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96
RLE Progress Report Number 140
Chapter 1. Optics and Quantum Electronics
Chapter 1.
Optics and Quantum Electronics
Academic and Research Staff
Professor Hermann A. Haus, Professor Erich P. Ippen, Professor James G. Fujimoto, Professor Peter L. Hagelstein, Dr. Brett E. Bouma, Dr. Jay N. Damask
Visiting Scientists and Research Affiliates
Dr. Mark E. Brezinski, Dr. Katherine L. Hall, Dr. JCirgen Herrmann, 2 Dr. Stefano Longhi, Dr. Mordehai Margalit,
Dr. Masayuki Matsumoto, Dr. Victor P. Mikhailov,3 Leo J. Missaggia, 4 Dr. Shu Namiki, Dr. Dominique S. Peter,
Dr. Carmen Puliafito,5 Dr. Joel Schuman, 6 Dr. Gunter Steinmeyer, Eric A. Swanson,7 Gaston Tudury, Dr. James
N. Walpole 8
Graduate Students
Igor P. Bilinsky, Stephen A. Boppart, Po-Hsiu Cheng, Seong-Ho Cho, Patrick C. Chou, David J. Dougherty,
John M. Fini, Matthew E. Grein, Boris Golubovic, Michael R. Hee, David J. Jones, Farzana I. Khatri, Mohammed J. Khan, Christina Manolatou, Lynn E. Nelson, Costantinos Pitris, Rohit Prasankumar, Daniel J. Ripin,
Susan Sujono, Guillermo J. Tearney, Erik R. Thoen, Constantine Tziligakis, William S. Wong, Charles Yu
Undergraduate Students
Ravindra V. Dalal
Technical and Support Staff
Mary C. Aldridge, Donna L. Gale, Cynthia Y Kopf
1.1
Modelocked Lasers using Erbium-
Project Staff
doped Glass Waveguide Amplifiers
David J. Jones, Professor Hermann A. Haus, Profes-
Sponsors
Defense Advanced Research Projects Agency
Grant F49620-96-0126
Scientific
Force
Air of
Office
Research
Scientific Research
of
-- Office
U.S. Air Force U.S.
Grant F49620-98-1-0139
sor Erich P. Ippen, Dr. Shu Namiki
In our past work on short-pulse fiber lasers, we have
developed both soliton and stretched-pulse fiber
lasers. 9 These fiber lasers, which operate in the 1.5
pm region, have utilized erbium-doped fiber (EDF) as
the gain element. Recently, erbium-doped planar
waveguide amplifiers have been developed as an
alternative to EDF. 10 With a typical length of 4.5 cm
these integrated optical amplifiers are able to deliver
1
2
3
Research Affiliate, Cardiac Unit, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts.
MIT Lincoln Laboratory, Lexington, Massachusetts.
International Laser Center, Polytechnical Academy, Minsk, Belarus.
4
MIT Lincoln Laboratory, Lexington, Massachusetts.
5
Chairman, Department of Ophthalmology, Tufts University Medical School and Director, New England Eye Center, Boston, Massachusetts.
6
Director, Glaucoma Services, New England Eye Center, Tufts New England Medical Center, Boston, Massachusetts.
7
MIT Lincoln Laboratory, Lexington, Massachusetts.
8
9
Ibid.
L.E. Nelson, D.J. Jones, K. Tamura, H.A. Haus, and E.P. Ippen, "Ultrashort-Pulse Fiber Ring Lasers," Appl. Phys. B65: 277 (1997).
D. Barbier, M. Rattay, F. Saint Andre, G. Clauss, M. Trouillon, A. Kevorkian, J.-M. P. Delavaux, and E. Murphy, "Amplifying Four-Wavelength Combiner, Based on Erbium/Ytterbium-Doped Waveguide Amplifiers and Integrated Splitters," IEEE Photonics Technol. Lett. 9:
315 (1997).
10
Chapter 1. Optics and Quantum Electronics
10 dB of gain at 1.53 tm with 980 nm pump powers
of 130 mW. In this project we investigated the operating characteristics and performance of a modelocked
soliton fiber laser using such a waveguide amplifier
as the gain element.11
The key advantage of using waveguide amplifiers in
modelocked fiber lasers is a sharp reduction in cavity
length, allowing fundamental repetition rates of 100300 MHz. Another advantage is the possible on-chip
fabrication of a wavelength-division-multiplexing
pump coupler to create a truly compact integrated
optical component for constructing modelocked fiber
lasers. But most importantly, a short cavity simultaneously reduces two parasitic effects that limit the
performance of soliton fiber lasers 12: resonant sideband formation and saturation of the polarization
additive pulse modelocking (P-APM) mechanism.
work is continuing on shorter cavities in an effort to
generate even shorter pulses at fundamental repetition rates greater than 500 MHz.
QWP HWP
QWP
Similar to long distance transmission systems, dispersive waves generated by periodic perturbations in
the laser cavity form resonant sidebands. In the case
of soliton fiber lasers, the pulse width is limited by
sideband generation when the soliton period Zo
approaches the cavity length Zc. Since Zo is propor2,
this condition effectively clamps the pulse
tional to tC
the cavity length to 1.3 m, (while
By
reducing
width.
strongly anomalous to
dispersion
keeping the net
energy)
generation of signifipulse
a
high
maintain
cantly shorter solitons becomes possible.
An additional effect that limits the peak power (and
hence the pulse energy) of soliton fiber lasers is saturation of the P-APM mechanism. This condition
occurs because of the interferometric nature of PAPM which has a sinusoidal dependence of transmission with peak power. As a result, most soliton
fiber lasers tend to multipulse at high pump powers.
Shortening the total fiber length to 1.3 m reduces the
net nonlinearity per pass, thus avoiding P-APM saturation and production of multiple pulses.
By incorporating a waveguide amplifier into a fiber
laser, the resulting short cavity (1.3 m of fiber) generated 113 fs solitons with a pulse energy of 160 pJ at
a fundamental repetition rate of 130 MHz. Presently,
11
12
98
-1.0
-0.5
0.0
0.5
1.0
Time (psec)
Figure 1. Cavity diagram of modelocked fiber laser
using a waveguide amplifier. QWP, quarter-wave
plate; HWP, half-wave plate; BTP, birefringent tuning
plate; WDM, wavelength-division-multiplexer. Inset is
an intensity autocorrelation of output pulse with sechfit (dashed line). The pulse-width is 113 fs.
1.2
Publications
Barbier, D., M. Rattay, F. Saint Andre, G. Clauss, M.
Trouillon, A. Kevorkian, J.-M. P. Delavaux, and E.
Murphy. "Amplifying Four-Wavelength Combiner, Based on Erbium/Ytterbium-Doped Waveguide Amplifiers and Integrated Splitters." IEEE
Photonics Technol. Lett. 9: 315 (1997).
Jones, D.J., S. Namiki, D. Barbier, E.P. Ippen, and
H.A. Haus. "116-fs Soliton Source Based on an
Er-Yb Co-doped Planar Waveguide Amplifier."
IEEE Photonics Technol. Lett. Forthcoming.
D.J. Jones, S. Namiki, D. Barbier, E.P. Ippen, and H.A. Haus, "116-fs Soliton Source Based on an Er-Yb Co-doped Planar Waveguide
Amplifier," IEEE Photonics Technol. Lett., forthcoming; D.J. Jones, S. Namiki, D. Barbier, E.P. Ippen, and H.A. Haus, "Passively Modelocked Fiber Laser using an Er-Yb Co-Doped Planar Waveguide Amplifier," paper presented at Optical Fiber Communications '98,
San Jose, California, February 22-28, 1998.
L.E. Nelson, D.J. Jones, K. Tamura, H.A. Haus, and E.P. Ippen, "Ultrashort-Pulse Fiber Ring Lasers," Appl. Phys. B 65:277 (1997).
RLE Progress Report Number 140
Chapter 1. Optics and Quantum Electronics
Jones, D.J., S. Namiki, D. Barbier, E.P. Ippen, and
H.A. Haus. "Passively Modelocked Fiber Laser
using an Er-Yb Co-Doped Planar Waveguide
Amplifier." Paper presented at Optical Fiber
Communications '98, San Jose California, February 22-28, 1998.
Nelson, L.E., D.J. Jones, K. Tamura, H.A. Haus, and
E.P. Ippen. "Ultrashort-Pulse Fiber Ring Lasers."
Appl. Phys. B 65: 277 (1997).
1.3
Environmentally-Stable StretchedPulse Fiber Laser
pulse. After filtering, the pulse propagates through
the anomalous dispersion ring constructed of PM
fiber in a linear polarization state and then is injected
back into the P-APM pigtail section. PM fiber in the
ring section ensures environmental stability in this
portion of the cavity.
OWPHWP
#1
r;lF
AM]
HWP
P #2
#2 -{
980nm
#1
"'- WDM #1
Output
BT Bnormal
Sponsors
Defense Advanced Research Projects Agency
Joint Services Electronics Program
U.S. Air Force - Office of Scientific Research
Project Staff
David J. Jones, Lynn E. Nelson, Professor Hermann
A. Haus, Professor Erich P. Ippen
Previous versions of our stretched-pulse fiber
lasers1 3 have been passively modelocked via polarization additive pulse modelocking (P-APM) in a selfstarting ring cavity with non-polarization maintaining
(PM) fiber. Such a configuration is sensitive to
mechanical and temperature variations which may
induce drifts in the polarization bias of the P-APM
and thus affect its environmental stability. Previously
reported environmentally-stable soliton fiber lasers
have used a linear topology and a Faraday mirror
(FRM) to eliminate linear bias drift. As a step towards
obtaining high powers and broader spectra, we have
developed an environmentally-stable stretched-pulse
fiber laser that uses a sigma cavity design.14
The cavity construction is shown in Figure 2. Pulse
shaping via P-APM occurs only in the pigtail section
and the FRM eliminates any P-APM environmental
dependence. Emerging from the P-APM transmission port, a portion of the pulse is tapped off via a
variable output coupler which provides a high quality
13
14
15
16
FR
FR
+3
Er fiber
dispersion
5T BTP
anomalous
dispersion
PM fiber
I
Figure 2. Stretched-pulse sigma cavity. WDM,
wave-length division multiplexer; FR, Faraday
rotator; BTP, birefringent tuning plate; AM, amplitude
modulator; QWP, quarter wave plate; HWP, half wave
plate; PM, polarization maintaining.
The sigma design is not a traveling-wave cavity
through its entire length. First-order reflections from
bulk elements in the linear section of the cavity can
form etalons which impede self-starting via modepulling. 15 Spatial hole-burning in the erbium-doped
fiber is an additional hindrance for self-starting.1 6 To
overcome these obstacles, a fiber pigtailed LiNbO 3
traveling-wave modulator is included in the ring section of the cavity for assistance with pulse start-up.
After modelocking is initiated by turning on the modulator and adjusting the wave-plates and filter, the
modulator is turned off and single pulse operation
remains. By relying on the modulator to provide pulse
start-up, a high-energy pulse state can be directly
obtained with diode pump levels of < 200 mW. This is
a significant aspect of this laser as it represents the
first high-power stretched-pulse erbium-doped fiber
laser which can be pumped by conventional laser
diodes.
L.E. Nelson, D.J. Jones, K. Tamura, H.A. Haus, and E.P. Ippen, "Ultrashort-Pulse Fiber Ring Lasers," Appl. Phys. B 65: 277 (1997).
D.J. Jones, H.A. Haus, L.E. Nelson, and E.P. Ippen, "Stretched Pulse Generation and Propagation," accepted to IEICE of Japan special issue on Ultrashort Pulse Technologies and their Applications, forthcoming; D.J. Jones, L. E. Nelson, H.A. Haus, and E.P. Ippen,
"Diode-Pumped Environmentally Stable Stretched-Pulse Fiber Laser," IEEE J. Select. Topics Quantum Electron. 3:1076 (1997); D.J.
Jones, L. E. Nelson, H.A. Haus, and E.P. Ippen, "Environmentally Stable Stretched Pulse Fiber Laser Generating 120 fs Pulses,"
paper CTuY3, Conference on Lasers and Electro-Optics '97, Baltimore, Maryland, May 18-23, 1997.
K. Tamura, J. Jacobson, E.P. Ippen, H.A. Haus, and J.G. Fujimoto, "Unidirectional Ring Resonators for Self-Starting Passively ModeLocked Lasers," Opt. Lett. 18: 220 (1993).
F. Krausz and T. Brabec, "Passive Mode Locking in Standing Wave Laser Resonators," Opt. Lett. 18: 888 (1993).
Chapter 1. Optics and Quantum Electronics
Following the previously described start-up procedure, this laser generates nanojoule pulses with 50
nm of spectral width that are compressible to sub100 fs. Environmental stability is verified by imposing
local temperature changes of +35 degrees Celsius at
various points on the fiber with a heat gun. Even
under these conditions the sigma laser remained stably modelocked. Mechanical perturbations appropriate to research-grade laser packaging also failed to
disrupt modelocking. Overall turn-key operation with
the sigma laser was observed over a period of
months. Presently we are working with Clark-MXR,
Inc. to develop this version of the stretched-pulse
laser into a commercial product.
1.4 High-Repetition Rate Laser Sources
Sponsors
Defense Advanced Research Projects Agency
U.S. Air Force - Office of Scientific Research
Project Staff
Matthew E. Grein, Dr. Mordehai Margalit, Professor
Erich P. Ippen, Professor Hermann A. Haus
Active modelocking is an attractive technique for
applications requiring synchronization of a laser to an
external clock. For an ultra-high speed telecommunications system employing time-division-multiplexed
(TDM) channels, the requirements for such a laser
source include repetition rates in excess of 10 Gbit/s,
pulsewidths on the order of one picosecond, and low
amplitude and timing jitter.
The pulsewidths expected from a fiber laser with
active modelocking alone, at 10 GHz, are on the
order of 3-4 picoseconds. This group"7 has previously shown theoretically1 8 and experimentally that
much shorter pulses can be achieved by taking
advantage of soliton formation in the fiber. Graduate
17
18
19
20
21
student David J. Jones has previously demonstrated
624 fs pulses at 5 GHz, and graduate student Matthew Grein has recently achieved 600 fs pulses at 10
GHz. These pulsewidths are shorter by a factor of 4
than those predicted by the Siegman-Kuizenga theory 9 and appear to be consistent with the theory of
Kartner.20 These are important results for two reasons: they verify that solitonic effects produce the
pulse shortening predicted by theory, and they demonstrate that we can directly generate sub-picosecond pulses at high repetition rates. In fact, 600 fs
pulses are the shortest pulses reported to date,
directly generated at 10 GHz.
In achieving these recent results, we have shown
that filtering plays an extremely important role in the
operation of the laser. Indeed, we have found that
using the broadest filters leads to the shortest pulses.
While that fact may not seem so surprising, we found
that, in addition, opening up the filter bandwidth did
not come at the expense of pulse-to-pulse amplitude
fluctuations. Here again, we find beneficial features
of soliton formation: soliton effects with the addition
of spectral filtering provide an intensity-dependent
loss. 2 1 The explanation is that shorter pulses have
the broadest spectra, thus see the most loss in the filter. The spectral loss discourages the buildup of a
single, very short pulse and encourages the equalization of pulse energies among the time slots.
An illustration of the laser output is given in Figure 3.
The autocorrelation displays a background-free 1.0
picosecond pulse and rf sidemode suppression
greater than 55 dB, indicating that the amplitude fluctuations are less than 0.0003%, or a possible pulse
dropout rate of 1 every 316200 pulses.
D.J. Jones, H.A. Haus, and E.P. Ippen, "Sub-Picosecond Solitons in an Actively Mode-Locked Fiber Laser," Opt. Lett. 2:1818 (1996);
M.E. Grein, A Regeneratively Modelocked, Fiber Ring Laser, M.S. thesis, Department of Electrical Engineering and Computer Science, MIT, September 1997.
H.A. Haus and Y.Silberberg, "Laser Mode Locking with Addition of Nonlinear Index," IEEEJ. Quantum Electron. QE-22: 325 (1986);
F.X. Kartner, D. Kopf, and U. Keller, "Solitary-pulse Stabilization and Shortening in Actively Mode-locked Lasers,"J. Opt. Soc. Am. B
12: 1927 (1996).
D.J. Kuizenga and A.E. Siegman, "FM and AM Mode Locking of the Homogeneous Laser. Part I: Theory," IEEE J. Quant. Electron. 6:
694 (1970).
G.R. Huggett, "Modelocking of CW Lasers by Regenerative rf Feedback," Appl. Phys. Lett. 13:168 (1968); M. Nakazawa, E. Yoshida,
and Y. Kimura, "Ultrastable Harmonically and Regeneratively Modelocked Polarization-Maintaining Erbium Fiber Laser," Electron. Lett.
30: 1603 (1994).
M. Nakazawa, K. Tamura, and E. Yoshida, "Supermode Noise Suppression in a Harmonically Modelocked Fibre Laser by Self-phase
Modulation and Spectral Filtering," Electron. Lett. 32: 461 (1996).
100 RLE Progress Report Number 140
Chapter 1. Optics and Quantum Electronics
1.5
Asynchronous Phase-Modulated
Optical Fiber Buffer
Sponsors
Defense Advanced Research Projects Agency
U.S. Air Force - Office of Scientific Research
Project Staff
David J. Jones, Dr. Katherine L. Hall, Professor Hermann A. Haus, Professor Erich P. Ippen
Figure 3. Autocorrelation and rf spectrum of the
laser output.
One of the challenges to maintaining active modelocking is uncontrolled detuning between the intracavity modulator and the laser cavity harmonic. Temperature variations cause a change in the effective
length of the fiber, leading to a change of the repetition rate of the laser relative to that of the modulator.
The allowable detuning between the modulator and
the cavity harmonic is on the order of tens of kilohertz. Without correcting for detuning, it has been
observed in the lab that modelocking is lost after only
a few minutes of operation. To correct for detuning in
the laser, we have constructed a regenerative 2
scheme. The key element of the regenerative
scheme is the clock extraction circuit (CEC)-comprised of a photodetector, amplifier, filter, and a
phase shifter-that regenerates the signal driving the
modulator from the harmonics of the laser itself. We
have shown that employing the CEC ensures that the
laser remains modelocked indefinitely. Future work
includes stabilizing the laser with active control of the
cavity length to achieve repetition-rate stability in the
effort to build a truly clock-synchronized laser source.
Thesis
Grein, M.E. A Regeneratively Modelocked, Fiber
Ring Laser M.S. thesis, Department of Electrical
Engineering and Computer Science, MIT, September 1997.
We demonstrated successful loading, storage, and
unloading of 5 Gbit/sec packets from an asynchronous phase-modulated optical fiber ring buffer.23 The
asynchronous mode of operation 24 offers several
practical advantages over synchronous operation 25
including tolerance of drifts in cavity length and
loaded packet characteristics as well as possible
elimination of clock recovery on incoming packets.
In essence, an optical fiber buffer of this type is an
actively modelocked fiber ring laser that is held below
threshold so that pulses cannot be spontaneously
created. However, if pulses, seeded by "ones" in an
input packet, are able to build up and suppression of
zeros" is maintained, the buffer can successfully
store the packet. Viewed in this light, it is not surprising that loading of packets into the buffer induces
relaxation oscillations (RO) that modulate the packet
intensity as it circulates around the cavity. The time
constant of the RO is the geometric mean of the
upper-state lifetime of the erbium-dopant (in the silica
host) and the photon lifetime in the resonator and
equals approximately 10 ms. The detrimental effects
of RO have limited the storage of previous optical
fiber ring buffers to 20 ms.
The variation in the average power of the circulating
packets, due to RO, can be minimized by optically
injecting a CW holding beam. With this technique,
either the packet or a CW signal is launched into the
buffer. By correctly adjusting the power of the CW
signal relative to the average packet power, the
injected photon number is conserved, thereby inhibiting any RO. In this experiment a holding beam is
22
M. Nakazawa, K. Tamura, and E. Yoshida, "Supermode Noise Suppression in a Harmonically Modelocked Fibre Laser by Self-phase
Modulation and Spectral Filtering," Electron. Lett. 32: 461 (1996).
23
D.J. Jones, K. L. Hall, H.A. Haus, and E.P. Ippen, "Asynchronous Phase-Modulated Optical Fiber Ring Buffer," Opt. Lett. 23: 177
(1998).
24
C.R. Doerr, H.A. Haus, and E.P. Ippen, "Asynchronous Soliton Mode Locking," Opt. Lett. 23: (1958); H.A. Haus, D.J. Jones, E.P.
Ippen, and W.S. Wong, "Theory of Soliton Stability in Asynchronous Modelocking," J. Lightwave Technol. 14: 622 (1996).
25
C.R. Doerr, W.S. Wong, H.A. Haus, and E.P. Ippen, "Additive Pulse Mode-Locking/Limiting Storage Ring," Opt. Lett. 19: 1747 (1994).
Chapter 1. Optics and Quantum Electronics
generated by a gain-switched semiconductor laser
which is injected into the buffer when packets are not
present. Incorporating the holding beam led to significantly enhanced storage times of least 157 ms
(1600 circulations). Limitations with our diagnostic
equipment prevented us from observing storage
beyond this limit.
Although we observed storage of the packet envelope for at least 157 ms, it is important to verify maintenance of the individual bits in the packet. A 10 bit
portion of the loaded packet displayed on a 50 GHz
sampling scope is given in Figure 4a, while Figure 4b
shows the same 10 bit portion after 450 roundtrips (a
(a)
storage time of 44.3 ms). To obtain the trace in Figure 4b, the sampling scope is triggered at the loading
event and the delay is set to 44.3 ms. The apparent
widening of the pulses in Figure 4b is caused by
oscilloscope timing jitter due to the long time delay
after the trigger event. Similar pulse broadening is
observed on a trace taken directly of the packets with
a 45 ms trigger delay, confirming oscilloscope jitter
as the source. In addition, the sampling scope failed
to hold its trigger beyond this point, preventing us
from obtaining a packet trace past a storage time of
44.3 ms.
(b)
1.0-
1.0-
0.5<U,
0.1350
nn.ll
II
0.0I
0.1355
0.1360
0.1365
I .
0.5
0.1 370
time (js)
44.32610
I
I
44.3265
I
44.327
44.3275
44.3280
time (ps)
Figure 4. High-speed sampling scope trace showing a portion of (a) the initial, loaded packet; (b) after the 450
circulations. Pulse broadening in the stored packet is caused by jitter from the sampling oscilloscope.
1.6 Optical Pulse Filtering with
Dispersion-Imbalanced Loop
Mirrors
Sponsors
Defense Advanced Research Projects Agency
Joint Services Electronics Program
National Science Foundation
U.S. Air Force - Office of Scientific Research
Project Staff
William S. Wong, Shu Namiki, Dr. Mordehai Margalit,
Professor Erich P. Ippen, Professor Hermann A.
Haus, Dr. Per P. Hansen, 26 Dr. Torben N. Nielsen 27
26
27
28
29
30
102
The nonlinear optical loop mirror (NOLM) 28 and the
nonlinear amplifying loop mirror (NALM) 29 have been
shown to possess optical switching properties 30 when
they are imbalanced by an asymmetric coupler and
by an asymmetrically placed gain element, respectively. They can be applied in a number of applications such as all-optical demultiplexing of data
streams 31 and passive modelocking of short-pulse
fiber lasers. 32
We have demonstrated experimentally the possibility
of constructing a new nonlinear optical Sagnac loop
that is imbalanced by dispersion. 3 3 The response of
this loop is significantly different from those of
NOLMs and NALMs-since dispersion acts only on
Bell Laboratories, Lucent Technologies, Holmdel, New Jersey.
Ibid.
N.J. Doran and D. Wood, "Nonlinear-optical Loop Mirror," Opt. Lett. 13: 56 (1988).
M.E. Fermann, F. Haberl, M. Hofer, and H. Hochreiter, "Nonlinear Demonstration of Optical Soliton Switching in an All-fiber Nonlinear
Sagnac Interferometer," Opt. Lett. 14: 754 (1990).
K.J. Blow, N.J. Doran, and B.K. Nayar, "Experimental Demonstration of Optical Soliton Switching in an All-fiber Nonlinear Sagnac
Interferometer," Opt. Lett. 14: 754 (1989).
RLE Progress Report Number 140
Chapter 1. Optics and Quantum Electronics
pulses, cw input light of arbitrary intensity is reflected
while short pulses are switched out. Our idea is to
construct the loop, as shown in Figure 5a, with one
segment of fiber of high anomalous dispersion D1 ,
and another segment with a much lower amount of
dispersion D2 . In the clockwise-propagating direction
inside the loop mirror, the N < 1 (N is the soliton
order) incident pulse disperses quickly and then
remains broad in the second segment, inducing little
nonlinear phase shift;
on the other hand, the
counter-clockwise-propagating pulse remains short
for the entire first segment where the fiber is almost
dispersionless, thus acquiring a large amount of nonlinear phase shift that is both intensity-dependent
and proportional to L2 (Figure 5b). It is important to
note that such nonreciprocal phase shift exists for
pulsed input but not for cw input. In other words, the
dispersion-imbalanced loop is a nonlinear pulse filter
that responds not to a dc input intensity, but rather to
the second derivative of the electric field envelope u
with respect to time.
The input and transmitted optical spectra are measured and shown in Figure 6. The extinction (loss) for
cw in the output, relative to that of the pulse output, is
22 dB. Moreover, the pulse spectrum is modified in a
favorable way-the spectral width broadens from 9
nm to 14 nm, while the resonant solitonic sidebands
are rejected by the loop, resulting in a shorter and
cleaner pulse after propagation in a DCF fiber. As
shown in Figure 7, the output FWHM pulse width is
compressed to 230 fs after 2.5 m of DCF, assuming a
sech shape (time-bandwidth-product = 0.40). The
suppression of the pedestal due to cw is evident in
the log plot of the pulse autocorrelations before and
after the loop.
31
32
33
34
In collaboration with Lucent Technologies in Holmdel,
New Jersey, we also demonstrated the possibility of
using a dispersion-imbalanced loop mirror to perform
nonlinear noise filtering on a 10 Gb/s noise-loaded
pulse stream. 34 In a fiber-optic communication system with in-line optical amplifiers, it is the presence of
amplified spontaneous emission (ASE) noise in the
signal stream that lowers the signal-to-noise ratio of
the signal at the optical receiver, resulting in a higher
bit error rate in transmission. By removing the inband ASE noise, the resulting receiver sensitivity is
improved from -37.0 dBm to -37.7 dBm. The experiment setup is shown in Figure 8. A 10 GHz pulse
train, where the 15 ps pulses have a time-bandwidthproduct of 0.35 at 1550 nm, is modulated at 10 Gb/s
by a lithium niobate amplitude modulator with a 231-1
pseudo-random bit sequence. The noise source consists of two erbium-doped amplifier stages in cascade, into which a 1 nm band-pass filter is inserted
between the two stages to lower the noise bandwidth
and increase the noise spectral power simultaneously. As a result, 8.3 dBm of ASE noise is generated at the signal wavelength with a bandwidth of 1.3
nm. The signal and the ASE noise are both launched
into the loop mirror via a 50/50 coupler followed by a
three-port optical circulator. The dispersion-imbalanced loop mirror is constructed with a 50/50 coupler, 12.62 km of Lucent TrueWave fiber with an
anomalous dispersion of +3.52 ps/(nm.km), and
11.36 km of TrueWave fiber with an anomalous dispersion of +1.68 ps/(nm - km) at 1550.0 nm.
D.M. Patrick, A.D. Ellis, and D.M. Spirit, "Bit-rate Flexible All-Optical Soliton De-multiplexing using a Nonlinear Optical Loop Mirror,"
Electron. Lett. 29: 702 (1993). K. Uchiyama, H. Takara, T. Morioka, S. Kawanishi, and M. Saruwatari, "100 Gbit/s Multiple-channel Output All-optical Demultiplexing based on TDM-WDM Conversion in a Nonlinear Optical Loop Mirror," Electron. Lett. 32: 1989 (1996).
I.N. Duling, Ill, "Subpicosecond All-fibre Erbium laser," Electron. Lett. 27: 544 (1991); D.J. Richardson, R.I. Laming, D.N. Payne, V.
Matsas, and M.W. Phillips, "Self-starting, Passively Modelocked Erbium Fibre Ring laser Based on the Amplifying Sagnac Switch,"
Electron. Lett. 27: 542 (1991); S. Wu, J. Strait, and R.L. Fork, "High-power Passively Mode-locked Er-doped Fiber Laser with a Nonlinear Optical Loop Mirror," Opt. Lett. 18:1444 (1993).
W.S. Wong, S. Namiki, M. Margalit, E.P. Ippen, and H.A. Haus, "Optical Pulse Filtering with Dispersion Imbalanced Nonlinear Loop
Mirrors," OFC '97 postdeadline paper PD-8, Dallas, Texas, 1997; W.S. Wong, S. Namiki, M. Margalit, E.P. Ippen, and H.A. Haus, "Selfswitching of Optical Pulses in Dispersion-imbalanced Nonlinear Loop Mirrors," Opt. Lett. 22: 1150 (1997).
W.S. Wong, P.B. Hansen, T.N. Nielsen, M. Margalit, S. Namiki, E.P. Ippen, and H.A. Haus, "In-band Amplified Spontaneous Emission
Noise Filtering with a Dispersion-imbalanced Nonlinear Loop Mirror," submitted to J. Lightwave Technol.
Chapter 1. Optics and Quantum Electronics
peak
intens ity
SMF
f
Area = Net Nonlinear Phase Shift
clockwise
SMF
DSF
distance
distance
(b)
(a)
Figure 5. (a) Conceptual operation of a dispersion-imbalanced loop mirror showing pulse stretching and
compression. (b) Plots of the pulse peak intensity as a function of distance along the two propagation directions.
10
.
0-
E- -
010
M
"-1
"- -20-30-
-o -.'
-40
0
0
.- 50O
060
-70
1500
1550
1600
Wavelength (nm)
Figure 6.
Input and output optical spectra.
104 RLE Progress Report Number 140
1500
1550
Wavelength (nm)
1600
Chapter 1. Optics and Quantum Electronics
1.0
0.1
Z0
r
0.01
0.5
U)
0.001
0.0
'I
......................
-8000
-6000 -4000
-2000
0
2000
4000
6000
8000
0.0001
I
-8000 -6000
" 111
'
-2000
0
2000
.11
4000
6000
8000
Time Delay (fs)
Time Delay (fs)
Figure 7.
P'
-4000
Autocorrelations of the input pulse
(TFWHM=
230 fs).
EDFA
Optical
Circulator
Reflected
Port
Figure 8. Experimental setup: PC, polarization controller; EDFA, erbium-doped fiber amplifer; BDF, bandpass filter.
For pulse input without noise injected, the transmitted power as a function of input power is measured
and shown in Figure 9. The switching energy corresponding to the first peak is 4 pJ, and the output
FWHM pulse width is shortened to 9 ps at that point.
The agreement with numerical simulations, computed with a split-step Fourier algorithm, is good. The
input and the transmitted signal through the loop mirror are both fed to an optically pre-amplified lightwave receiver for bit error rate measurements.
Without noise loading, the measured BERs through
the loop mirror at 28 mW of input power are shown in
Figure 10 as circles. When compared to the BERs
without the loop mirror (squares), one can see that
the insertion of the loop mirror causes no penalty in
receiver sensitivity. Furthermore, when in-band ASE
noise is added to the system such that the input optical signal-to-noise ratio decreases from 38 dB to 15
dB in 0.2 nm of bandwidth, the degraded sensitivity
Chapter 1. Optics and Quantum Electronics
of -37.0 dBm measured without the loop mirror (triangles) can be improved to -37.7 dBm (inverted triangles).
sion Imbalanced Nonlinear Loop Mirrors." OFC
'97 postdeadline paper PD-8, Dallas, Texas,
1997.
Wong, W.S., S. Namiki, M. Margalit, E.P. Ippen, and
H.A. Haus. "Self-switching of Optical Pulses in
Dispersion-imbalanced Nonlinear Loop Mirrors."
Opt. Lett. 22: 1150 (1997).
1.7
Stretched-Pulse Propagation
Sponsor
U.S. Air Force - Office of Scientific Research
Project Staff
Professor Hermann A. Haus, Dr. Masayuki Matsumoto
10
20
30
Linear non-return-to-zero (NRZ) long-distance communications without repeaters has been in place
since 1995. Competing nonlinear soliton communications has not yet won a competitive edge for the following reasons:
Input Power (mW)
Figure 9. Switching characteristics of the
dispersion-imbalanced loop mirror.
1.
Long distance soliton communication systems
have been based on the sliding guiding filter
principle.35 Whereas the performance of these
systems is greatly superior to the "linear"
approach, the sliding guiding filters require different filters in different amplifier pods. This
feature seems unacceptable to system engineers.
2.
NRZ can span the Pacific with a bit-rate of 5
Gbit/s and allows expansion with wavelength
division multiplexing.
-7 -8
-9 -10 -11 -12 -42
-41
-40
-39
-38
-37
-36
Received Power (dBm)
Through loop mirror
Noise-loaded signal through loop mirror
Back-to-back
Noise-loaded signal
Figure 10. Measured bit error rate curves as a
function of power received at the receiver.
1.6.1
Publications
Wong, W.S., S. Namiki, M. Margalit, E.P. Ippen, and
H.A. Haus. "Optical Pulse Filtering with Disper35
36
However, nonlinear schemes of propagation accommodate, to a great extent, the nonlinear properties of
fibers and allow for a higher S/N ratio. We have been
studying the propagation of pulses of high intensity in
systems of alternating negative and positive dispersion fiber segments. The pulses alternately stretch
and compress within each "cell" of a pair of positively
and negatively dispersive fiber segments. In this
respect, the propagation is similar to the propagation
of pulses within the passively modelocked stretched
pulse laser.3 The stretched pulse laser operates via
self-phase modulation, self-amplitude modulation, filtering and group velocity dispersion. In stretched
pulse communications there is no need for self-
L.F. Mollenauer, J P. Gordon, and S.G. Evangelides, "The Sliding Frequency Guiding Filter: an Improved form of Soliton Jitter Control,"
Opt. Lett. 17: 1575 (1992).
M. Matsumoto and H.A. Haus, "Stretched-pulse Optical Fiber Communications," IEEE Photonics Technol. Lett. 9: 785 (1997).
106 RLE Progress Report Number 140
Chapter 1. Optics and Quantum Electronics
amplitude modulation required in a laser for pulse
stability against noise build-up. Stretched pulse propagation can occur in the regime of negative as well
as positive average dispersion. In the latter case, the
pulses are chirped. The scheme has several advantages.
The average dispersion can be set by adjustment of the individual fiber segment lengths
and is not determined by the availability of fiber
of a particular dispersion characteristic. The
average dispersion can be set at a much lower
value than could be achieved with uniform fiber
under existing fabrication tolerances. Small
dispersion reduces the Gordon-Haus jitter38
and thus it is possible to attain acceptable
transmission characteristics even without sliding guiding filters.
The average dispersion can be positive as
well. This permits channel distributions over a
wider bandwidth than would be possible if
propagation were limited to net negative dispersion.
Since the pulses are not solitons and are
chirped, phase matching to sidebands of perturbations due to periodic amplification is
reduced, permitting a greater (more than triple)
amplifier spacing than that of soliton systems.
1.
2.
3.
We are pursuing the study of these systems in particular with respect to pulse collisions on bit-error rates
of wavelength division multiplexed systems.
1.7.1
Publication
Matsumoto, M., and H.A. Haus. "Stretched-Pulse
Optical Fiber Communications." IEEE Photonics
Technol. Lett. 9: 785 (1997).
37
38
39
40
1.8
Direct Measurement of Self-Phase
Shift due to Fiber Nonlinearity
Sponsors
National Science Foundation
U.S. Air Force - Office of Scientific Research
U.S. Navy - Office of Naval Research
Project Staff
Dr. Mordehai Margalit, Charles Yu, Professor Hermann A. Haus, Professor Erich P. Ippen
Many nonlinear optical experiments utilize glass
fiber. One of the most important nonlinear effects
experienced by a pulse propagating through the fiber
is the accumulation of a nonlinear phase shift. Thus,
this nonlinear phase shift is a good measure of the
fiber nonlinearity. Moreover, for some experiments
this phase directly controls the experimental outcome. 39 Knowledge of the nonlinear phase shift is
essential. Most previously used measurement techniques only characterize the dynamic phase within
the pulse and do not determine the absolute phase
shift.
We have devised a novel method to measure this
phase shift based on spectral interferometry (SI). 4 0 SI
measures the sum spectrum of a reference and a
signal that are coherent and separated in time by t.
The beating between signal and reference generates
a sinusoid in frequency whose period is the inverse
of the temporal separation t. We ensure that only the
signal, not the reference, is affected by the fiber nonlinearity. So the nonlinear phase imposed on the signal distorts the signal-reference beating sinusoid.
Near the center of the spectrum the distortion is manifested by a shift of the beating pattern, and the shift
is a direct measure of the phase at the peak wavelength. We have shown numerically that it approximates well the nonlinear phase shift at the temporal
peak of the pulse.
H.A. Haus, K. Tamura, L.E. Nelson, and E.P. Ippen, "Stretched Pulse Additive Pulse Mode-Locking in Fiber Ring Lasers; Theory and
Experiment," J. Quant. Electron. 31: 1-8 (1995).
J.P. Gordon and H.A. Haus, "Random Walk of Coherently Amplified Solitons in Optical Fiber Transmission," Opt. Lett. 11: 665-67
(1986).
K. Bergman and H.A. Haus, "Squeezing in Fibers with Optical Pulses," Opt. Lett. 16: 663 (1991); W. Wong, S. Namiki, M. Margalit,
H.A. Haus, and E.P. Ippen, "Self-Switching of Optical Pulses in Dispersion-Imbalanced Nonlinear Loop Mirrors," Opt. Lett. 22: 1150
(1997).
C. Froehly, A. Lacourt, and J. Vienot, "Time Impulse Response and Time Frequency Response of Optical Pupils: Experimental Confirmations and Applications," J. Opt. (Paris) 4:183 (1973).
107
Chapter 1. Optics and Quantum Electronics
These experimental observations agree well with our
theoretical calculations and numerical simulations.
Figure 11 shows some of the experimental results.
Figure 11a shows the optical spectrum of the pulse
pair before and after the fiber when dispersion is
negative. Figure 11b shows the same spectra on an
expanded 1 nm scale. The fringes are uniform and
the phase shift can be easily deduced. Figure 11c
and Figure 11d show the spectra for the positive dispersion case with different spans. The fringes are
again uniform. This technique is easy to implement
and very accurate. We expect it to be a useful diagnostic tool for many nonlinear fiber experiments.
We have implemented this technique in an experiment using 1.7 km of fiber. We tuned our optical
source so that the fiber dispersion was negative or
positive depending on the wave-length of the optical
signal. We observed that for the positive dispersion
case the spectrum broadens as the input power
increases. The nonlinear phase shift also increases
linearly with input power. For the negative dispersion
case, the spectrum narrows as the input power increases because of soliton effects. The nonlinear
phase shift also increases initially. However, when
the soliton condition is reached, both the spectral
bandwidth and the nonlinear phase shift stay the
same. This is because an increase of the input power
beyond the soliton limit generates continuum and
does not contribute to the soliton energy.
90 WI
I
I
1573.6
1573.8
(b)
60
W
601574.0
60pW
1574.4
1574.2
Input
,
-60
I
1573.6
,
I
I
,
,
1574.0
1573.8
I
I
1574.2
1574.4
1574.2
1574.4
Input
-70
-80
1565
1570
1575
1580
1585
-26
Wavelength, nm
,L
1573.6
1574.0
1573.8
Wavelength, nm
-25
104'W
-30
(d)
-
-35
52gW
-34
-35
-36
26W
Input
-37
-38
1532 1534 1536 1538 1540 1542 1544 1546 1548 1550 1552
Wavelength, nm
1540.8 1540.9 1541.0 1541.1 1541.2 1541.3 1541.4 1541.5 1541.6
Wavelength, nm
fiber, 1 nm span for
Figure 11. (a) Spectra before and after fiber, 20 nm span; (b) Spectra before and after
(d) Spectra
span;
nm
20
fiber,
the
after
and
different powers, negative dispersion case; (c) Spectra before
case.
dispersion
positive
powers,
different
for
span
nm
1
before and after fiber,
108 RLE Progress Report Number 140
Chapter 1. Optics and Quantum Electronics
1.9 Wavelength Stabilized Modelocked
Laser Source Gyro Applications
Sponsor
Charles S. Draper Laboratory
Project Staff
Patrick C. Chou, Dr. Oldrich M. Laznicka, Professor
Hermann A. Haus
We are investigating techniques for generating optical radiation suitable for exciting a high-performance
fiberoptic gyroscope and other fiber-based sensors.
An ideal source would be continuous wave, high
power, very low in coherence, and with a mean
wavelength stable to better than a part per million.
Low coherence is necessary to minimize errors due
to backscattering. Since measurements can occur
over periods of days or weeks, wavelength stability is
required because the measured signal must not drift
over that time.
The conventional scheme uses a superluminescent
diode, in which it is difficult to achieve high power
and low coherence simultaneously, especially at 1.55
ptm where superluminescent diode technology is relatively undeveloped. The basis of our optical source
is a stretched pulse fiber laser, which is attractive
because of its wide optical bandwidth and relatively
high power. The wide bandwidth translates into low
coherence. This laser has high power output, unlike
the superluminescent diode which tends to narrow in
bandwidth as power is increased.
One drawback to the stretched pulse fiber laser is
that its output pulses have very high peak intensity
and result in errors due to optical nonlinearity in the
fiber of the gyro coil. We have shown that this can be
alleviated by properly preparing the pulses before
they enter the gyro. One must send them through a
highly dispersive medium so that the pulse widths
become so long that they overlap each other. It may
also be necessary to scramble the phases of the
pulses to prevent beats.
Mean wavelength stabilization is also a critical issue.
Stabilizing a broadband optical source is different
from the more common problem of stabilizing a single frequency laser. First, one must accurately sense
the mean wavelength, analogous to the center of
mass of the spectrum. A compact and stable device
to measure this quantity is currently being custom
manufactured in a glass waveguide structure. Sec-
ond, one must be able to control the mean wavelength. Since we are working with chirped pulses, we
can take advantage of the fact that the different frequencies of a chirped pulse are spread out over time.
We can then control the optical spectrum by modulation in the time domain.
For the fibergyro application, low-frequency noise is
also an issue because the detection schemes incorporate modulation at frequencies from 100 kHz to a
few MHz. It is not clear yet whether it is easier to
build a high-power, low-noise pulse source or an
amplified low-power, low-noise source. We are currently investigating both cases.
1.10 Optical Measurements of Photonic
Bandgap Resonators in the Near
Infrared
Sponsors
National Science Foundation/MRSEC
U.S. Air Force - Office of Scientific Research
Project Staff
Daniel J. Ripin, Dr. Brent E. Little, Dr. Gunter Steinmeyer, Erik R. Thoen, Professor Erich P. Ippen
Photonic bandgap crystals (PBGs) are periodic lattices comprised of materials with very different
dielectric constants. These crystals can be used for
the basic manipulation and control of light, similar to
the ways that electronic crystals manipulate electrons. With the addition of defects, photonic crystals
can confine light to dimensions on the order of its
wavelength. One-dimensional dielectric structures
utilizing these unique properties have been designed
to serve as narrow bandpass wavelength filters. The
basic design is an optical ridge waveguide with
evenly spaced air holes. A defect in the spacing
between two of the holes is introduced to create a
high-Q cavity. Light on resonance with the cavity is
transmitted, while all other wavelengths are reflected.
Structures with this design have been fabricated in
two material systems, Si/SiO 2 and GaAs/AIxOy. Filters in the proper wavelength regime have the potential to be applied as channel dropping filters for
WDM, or as components of integrated photonic chip
devices.
We have characterized various devices designed
and fabricated in collaboration with the groups of
Professors Joannopoulos, Kimerling, Kolodziejski,
and Smith and observed for the first time a PBG res-
109
Chapter 1. Optics and Quantum Electronics
onance in the near IR. A cw tunable NaCI F-center
laser was used for the transmission measurements.
This laser is tunable from 1510 nm to 1680 nm, with
an average output power of 250 mW. The light is
coupled into small ridge waveguides with dimensions
0.2 x 0.5 ptm using an optical fiber with a lensed output tip. Figure 12 shows the transmission measured
through the PBG device in the silicon system.41 The
resonance is at 1560 nm, within 1% of the calculated
value 1547 nm, and has a quality factor of 265. The
modal volume was calculated to be 0.055 pm 3. By
using structures with scaled hole dimensions, we
were able to observe the band edges of the gap. A
shift was then calculated to put the band edges on
the same plot as the resonance, giving excellent
agreement with theory.
1.0
-
-I
S0.6
E
Foresi, J.S., P.R. Villeneuve, J. Ferrera, E.R. Thoen,
G. Steinmeyer, S. Fan, J.D. Joannopoulos, L.C.
Kimerling, H.I. Smith, and E.P. Ippen. "PhotonicBandgap Microcavities in Optical Waveguides."
Nature 390: 143 (1997).
0.4
--
0.2
)I
1300
1.11 Optical Resonant Structures
'
':
0.0
1400
1500
160 0
1700
Wavelength (nm)
Figure 12. Comparison of measured transmission
(solid lines) to calculated transmission (dotted line)
for a PBG microcavity with four holes on each side of
the microcavity. The upper-band and lower-band
edge transmission data have been shifted in
wavelength according to hole dimension scaling.
Using the same experimental setup, we have also
studied transmission through ring resonators, a
promising alternative to PBG devices for wavelength
filtering. The ring resonator, two high dielectric contrast waveguides side-coupled to a circular ring
waveguide, was also manufactured in the silicon
material system and measured. 4 2 The waveguide
dimensions are the same as above but are fabricated
with polycrystalline silicon. These waveguides were
measured to have scattering losses around 20 dB/
cm. The fabricated rings have radii ranging from 3 to
41
42
110
PBG devices using GaAs structures are also being
fabricated in Professor Kolodziejski's laboratory.
These devices include air-bridge structures as well
as the ridge geometry discussed above. In air-bridge
crystals, the PBG is suspended over air, which will
increase the dielectric contrast between the guide
and surrounding material and produce even tighter
resonator mode confinement. Coupling to these
bridges and waveguide transmission losses is being
investigated experimentally. New experiments are
also underway to further develop the ring resonator
technology.
1.10.1 Publication
Measured data (scaled)
.........
Calculated
0.8
5 mm. Several resonances were observed near 1550
nm with a quality factors up to 250 and free spectral
ranges (FSR) as large as 24 nm. The depths of the
nulls were measured to be deeper than 15 dB down
from the off-resonance values.
Sponsors
Defense Advanced Research Projects Agency
National Science Foundation/MRSEC
Project Staff
Mohammad Jalal Khan, Christina Manolatou, Dr. Jay
N. Damask, Dr. Shan-Hui Fan, Dr. Brent E. Little,
Professor Hermann A. Haus
We are investigating the design and modeling of resonant structures for use as channel dropping filters
that are key components for WDM. These filters
access one particular channel of a WDM signal without disturbing other channels. The linewidth of channel dropping filters must be wide enough to accommodate the bandwidth of the individual channels
(about 10 GHz) but also narrow enough to avoid
cross talk from adjacent channels (spaced about 100
GHz apart). In order to accommodate the full band-
J.S. Foresi, P.R. Villeneuve, J. Ferrera, E.R. Thoen, G. Steinmeyer, S. Fan, J.D. Joannopoulos, L.C. Kimerling, H.I. Smith, and E.P.
Ippen, "Photonic-Bandgap Microcavities in Optical Waveguides," Nature 390: 143 (1997).
B.E. Little, J.S. Foresi, G. Steinmeyer, E.R. Thoen, S.T Chu, H.A. Haus, E.P. Ippen, L.C. Kimerling, W. Greene, "Ultra-Compact Si/
SiO 2 Microring Resonator Optical Channel Dropping Filters," IEEE Photonic Tech. Lett., forthcoming.
RLE Progress Report Number 140
Chapter 1. Optics and Quantum Electronics
width of the erbium doped fiber amplifiers (about 40
nm around the 1550 nm optical wavelength) these filters must also have large free spectral range (FSR).
Resonant structures can achieve very narrow linewidths for a given device size and can be combined
together to further modify the Lorentzian response of
a single resonator yielding higher order filters. Placed
between a bus and a receiver waveguide, they can
tap off all the power of a particular channel from the
bus, at the resonance frequency, and transfer it to the
receiver. In order to satisfy the WDM system requirements the loaded, and the radiation Q's of the resonators should be of the order of 103 and 104,
respectively, and their size should be small in order
to achieve a large FSR. The use of resonators based
on high index contrast structures may be a viable
approach leading to device sizes only of the order of
a few wavelengths.
We are investigating a type of channel dropping filter
that employs degenerate symmetric and antisymmetric resonant modes. The simplest realization of the
basic structure is a ring resonator. The ring supports
two modes that can be described either in terms of
circulating waves in opposite directions around the
ring or in terms of standing waves. The main difficulty
in the use of ring resonators is associated with the
surface roughness of the ring waveguide walls,
which, in addition to causing radiation loss, couples
the two counter propagating traveling waves and
degrades the performance.
A rough estimate can be obtained analytically, but
the actual performance can only be determined from
a full numerical solution of Maxwell's equations which
includes the loading of the resonances. We use the
finite difference time domain (FDTD) method to find
the resonator modes, the radiation and loaded Q's,
and the spectral response of the filter over a wide
bandwidth for different material and geometrical
parameters of the waveguides and resonators.
A square or rectangular dielectric structure is a very
simple realization of a standing wave resonator. In
such structures most of the radiation loss is expected
at the corners, so a mode with a high Q should have
nulls at the corners. Figure 13 shows the field computed using FDTD in a square resonator side coupled to a waveguide, after elapse of time so that the
mode with the highest Q has survived. Of all the
modes that are initially excited in the resonator by the
waveguide field, the ones with high field intensity at
the corners, soon radiate away leaving the mode of
Figure 13 that has nulls along the diagonals of the
square. The ultimate aim is to construct a degenerate
symmetric and antisymmetric mode of two square or
rectangular resonators via side-coupling to two
waveguides as shown in Figure 14.
The principle of operation of the ring resonator can
be generalized to standing wave resonant structures
with symmetry, such as a pair of mutually coupled
identical standing wave resonators. When these
modes are side-coupled to two waveguides, full
power transfer can be achieved at resonance provided that the symmetric and antisymmetric modes
have the same resonance frequency and quality factor.
Coupling of modes is our first approximation for analyzing the filter performance and for deriving an
equivalent circuit which provides a correspondence
with standard filter design techniques. This approach
can further be used for design of multipole filters in
the case that the 20 dB/decade roll-off of the single
pole Lorentzian response is not adequate to meet the
cross-talk specifications. (This is done by coupling
several pairs of degenerate resonators; the coupling
between the resonators is appropriately chosen to
obtain the desired spectral response.)
Figure 13. The electric field intensity of the mode
of highest Q in a square cavity.
Chapter 1. Optics and Quantum Electronics
*-*
E -riqI
-'-:gn,.l q Bus
Figure 14.
filter.
I
C:'~ L
p
I
A coupled cavity channel drop- or add-
1.12 Ultrafast Gain-Index Coupling in
InGaAsP Diode Lasers
Sponsors
Joint Services Electronics Program
U.S. Air Force - Office of Scientific Research
Project Staff
Erik R. Thoen, Dr. Gunter Steinmeyer, Po-Hsiu
Cheng, Daniel J. Ripin, Dr. Katherine L. Hall, Dr.
Joseph P. Donnelly, Professor Erich P. Ippen
Diode lasers and semiconductor optical amplifiers at
telecommunication wavelengths are being used
increasingly for high-speed data transmission, optical
switching, four-wave mixing, and other nonlinear
optical applications. A basic understanding of their
nonlinear properties on ultrafast time scales is
essential to define the limits of the current technologies and to define new applications. We have performed heterodyne pump-probe measurements to
determine the ultrafast response of InGaAsP quantum well semiconductor optical amplifiers at 1.55 pm.
A femtosecond optical parametric oscillator system
has been used to study the wavelength dependence
of the various contributions to the ultrafast response
in these devices.
An experimental investigation of dynamic gain-index
coupling in such amplifiers has been performed
using simultaneous measurements of transmission
and phase changes. Both gain and index dynamics
are expected to contribute artifacts to each other's
measurements proportional to the time derivative of
the response and the spectral slope of the linear
transmission function. Typical experimental traces of
the apparent gain and index dynamics are shown in
Figure 15. The deduced contribution of the dynamic
112
RLE Progress Report Number 140
coupling, shown by the dashed curve, qualitatively
exhibits the expected dependences. The measurements suggest, however, that even when the gainindex coupling is accounted for, a delay in carrier
heating is still a significant component in the gain
response as indicated in our previous work.
Because our heterodyne technique is capable of
both gain and index measurements, careful measurements of the alpha parameters associated with
intraband density changes and interband carrier
heating have been obtained as a function of current
injection and wavelength. This is the first time that
the alpha parameter associated with carrier heating
has been measured as a function of wavelength in
these devices.
Currently we are optimizing a fiber laser based upon
the erbium-doped stretched-pulse fiber laser to be
used for sub-100 femtosecond measurements of
laser diodes. With such short pulses, the effect of
dynamic gain-index coupling should be even more
significant, and the time constant of carrier-carrier
scattering can be more accurately measured.
.000
-00D5
(a)
area
.500
90
0
1500
1000
Delay Isc)
0 004
0,002
0.000
. .........
.0002
-.004
(b)
-0.006
•500
. . ..
0
500
1000
1500
Delay osec)
Figure 15. The measured gain (top) and index
(bottom) response of an optical amplifier. In the top
graphic, the dotted line is the extracted component of
the gain response due to dynamic gain-index
coupling.
Chapter 1. Optics and Quantum Electronics
1.13 Femtosecond Studies of THz
Acoustic Phonons in PbTe Quantum
Dots
Sponsors
Joint Services Electronics Program
U.S. Air Force - Office of Scientific Research
Project Staff
Acoustic phonons in quantum dots have been analyzed theoretically by several authors. Using the bulk
velocity of sound in PbTe, we calculate a resonance
frequency of 0.4 THz for the fundamental breathing
mode of a 4.3 nm unstrained quantum dot. This frequency agrees qualitatively with what is observed
experimentally. We are in the process of investigating
the wavelength dependence of the oscillations and
the damping and relating them to the distribution,
dephasing, and other loss mechanisms.
Erik R. Thoen, Gaston Tudury, Dr. GOnter Steinmeyer, Daniel J. Ripin, Dr. Patrick Langlois, Professor
Erich P. Ippen
Because of their strong quantum confinement, semiconductor quantum dots offer the potential of application as efficient nonlinear optical elements. Recently,
their use for saturable absorber modelocking has
been successfully demonstrated. Previous work on
the nonlinear response of quantum dots has been
mainly concentrated on cadmium-compound nanocrystals, which exhibit strong nonlinearities in the visible spectrum. The lower bandgap of lead compounds
provides opportunity to produce quantum dots absorbing in the wavelength range from 1 to 2 pm, so
they can be tailored to the gain spectra of virtually all
solid state lasers in that range. Also, with their large
bulk exciton Bohr radius, they allow strong quantum
confinement with relatively large nanocrystals.
We are investigating the nonlinear transmission characteristics of PbTe quantum dot glasses (obtained
through collaboration with C.L. Cesar of UNICAMP,
Campinas, Brazil) with a high-sensitivity heterodyne
pump-probe experiment. A femtosecond optical
parametric oscillator system producing pulses of 130
fs, tunable from 1.35 to 1.6 pm, was used in these
experiments. The nonlinear response of a quantum
dot sample with an exciton peak at 1.365 pm excited
with 1.5 tm pump beam is shown in Figure 16. It
exhibits three components, a slow exponential decay
with a response time of 70 ps, a fast response with a
time constant in the range of 100 fs, and an oscillatory component with a frequency of 0.55 THz and
damping time constant of about 2.5 ps. The frequency of these oscillations is clearly a signature of
acoustic vibrations. To our knowledge, this is the first
time that coherent acoustic phonons have been
observed in quantum dots in a glass matrix.
43
MIT Lincoln Laboratory, Lexington, Massachusetts.
0.006
0.004
.
2
0002
1CCO
-1000
0
1i=
2000
2000
3000
3,d4
-.
4(
0M0
6000
Figure 16. Transmission of the probe at 1.5 mm as
a function of time through PbTe quantum dots in a
glass matrix. The oscillations result from excitation of
the spheroidal acoustic mode of the quantum dots.
1.14 Ultrashort Pulse Generation and
Ultrafast Phenomena
Sponsors
Joint Services Electronics Program
Grant DAAH04-95-1-0038
U.S. Air Force - Office of Scientific Research
Contract F49620-95-1-0221
U.S. Navy - Office of Naval Research (MFEL)
Contract N00014-91-J-1956
Contract N000014-97-1-1066
Project Staff
Dr. Brett E. Bouma, Igor P. Bilinsky, Seong-Ho Cho,
Boris Golubovic, Rohit Prasankumar, Professor Erich
P. Ippen, Dr. James N. Walpole, 43 Leo J. Missaggia, 44
Dr. Victor P. Mikhailov,4 5 Professor James G. Fujimoto
Chapter 1. Optics and Quantum Electronics
1.14.1 Novel Modelocked Laser Cavity
Designs For High-Peak Intensities
Ultrashort pulse sources are of fundamental importance for advances in signal processing, high speed
communications, and investigation of ultrafast nonlinear processes in semiconductor materials and
devices. Generally, these sources must be technologically simple, robust, and cost effective. In the last
several years, significant advances have been made
in the development of Kerr lens modelocking (KLM)
utilizing electronic Kerr effect for fast saturable
absorber action. KLM has allowed generation of the
shortest pulses ever produced directly from a laser
oscillator.46 Working in collaboration with Professors
Erich P. Ippen and Hermann A. Haus, we have developed a theoretical model which provides a foundation for understanding and optimizing short-pulse
KLM lasers.47
Generation of femtosecond pulses with high intensities in the MW range is essential for a number of
applications including optical harmonic generation
and investigation of ultrafast nonlinear optical phenomena. The peak power directly generated by modelocked Ti:AI 2O3 laser sources is in the range of 0.1
MW which is often insufficient for studies of nonlinear
phenomena. Along with a traditional short pulse
amplifier approach, our group has recently demonstrated the use of cavity dumping to increase output
pulse peak power from a KLM Ti:AI20 3 laser.48 Investigators have also examined techniques to design
standard repetition rate KLM lasers to operate at high
power and achieve high-intensity pulses. 49 The
development of low cost, high intensity laser sources
will enable a wider range of femtosecond measure-
44
45
46
47
48
49
50
51
114
ment applications, making this technology more
available to both the research and the development
community.
We focused our efforts on designs that generate high
intensity pulses directly from a modelocked laser
oscillator without the need for amplification. Although
these methods will not achieve pulse energies as
high as those obtained using traditional short pulse
amplifiers, they offer a significant advantage in cost
and system complexity.
We developed a simple alternative method for
improving peak power from a laser by designing a
novel long cavity geometry for KLM. Our objective is
to increase the laser output pulse energies and intensities by increasing laser cavity length. While keeping
the laser average output power constant, the pulse
energy can be increased by reducing the laser repetition rate. The development of long-cavity lasers
requires careful design because the operation of
KLM depends critically on laser cavity design. The
laser cavity must be operated in a particular subset of
its stability region in order to optimize modelocking
performance.
As an approach to developing long cavity lasers, we
explored the use of Herriott style multiple pass cavity
(MPC).5 o The MPC is constructed by a pair of curved
mirrors separated by a given distance. The optical
beam is introduced into the MPC, strikes the first mirror off center, is subsequently reflected between the
two mirrors in a circular pattern, and can be extracted
after a given number of round trips. The beam is also
focused on subsequent reflections so that its propagation resembles propagation through a periodic lens
array. This device is designed such that it provides a
unity transformation of the q parameter of the laser
beam after a given number of transits. 51 Thus, if this
MIT Lincoln Laboratories, Lexington, Massachusetts.
Professor, International Laser Center, Polytechnical Academy, Minsk, Belarus.
I.D. Jung, F.X. Kdrtner, N. Matuschek, D.H. Sutter, F. Morier-Genoud, G. Zhang, and U. Keller, "Self-starting 6.5-fs Pulses from a
Ti:sapphire Laser," Opt. Lett. 22: 1009-11 (1997).
H.A. Haus. J.G. Fujimoto, and E.P. Ippen, "Structures for Additive Pulse Modelocking," J. Opt. Soc. Am. B 8: 2068 (1991); H.A. Haus,
J.G. Fujimoto, and E.P. Ippen, "Analytic Theory of Additive Pulse and Kerr Lens Mode Locking," IEEE J. Quant. Electron 28: 2086
(1992).
M. Ramaswamy, M. Ulman, J. Paye, and J.G. Fujimoto, "Cavity-dumped Femtosecond Kerr-lens Mode-locked Ti:A120 3 Laser," Opt.
Lett. 18: 1822-24 (1993).
L. Xu, G. Tempea, A. Poppe, M. Lenzner, C. Spielmann, F. Krausz, A. Stingl, and K. Ferencz, "High-power Sub-10-fs Ti:sapphire Oscillators," Appl. Phys. B 65: 151-59 (1997).
D. Herriott, H. Kogelnik, and R. Kompfner, "Off-Axis Paths in Spherical Mirror Interferometers," Appl. Opt. 3: 523-26 (1964); B. Perry,
R.O. Brickman, A. Stein, E.B. Treacy, and P. Rabinowitz, "Controllable Pulse Compression in a Multiple-pass Raman Laser," Opt. Lett.
5:288 (1980).
12 8 38
- .
H.A. Haus, Waves and Fields in Optoelectronics (Englewood Cliffs, New Jersey: Prentice-Hall, 1984), pp.
RLE Progress Report Number 140
Chapter 1. Optics and Quantum Electronics
device is inserted into the KLM laser, it can be
designed such that it has a zero effective length and
leaves the laser cavity mode and nonlinear focusing
behavior invariant.
Following this approach, we designed a high peak
power laser using a standard, dispersion compensated KLM Ti:A120 3 resonator with a MPC incorporated into one of the arms. We used a design for
which the beam made 20 round trips between the
MPC mirrors. Stable KLM modelocking was achieved
resulting in 20 fs pulses at 15 MHz repetition rate,
compared to a repetition rate of 100 MHz typical for
conventional resonator geometries. This resulted in a
higher peak power for a given average output power.
Furthermore, because the pulse repetition rates are
significantly lower, parasitic thermal effects in
ultrafast measurements using this source will be significantly smaller than for conventional high repetition
rate 100 MHz lasers.
A limitation on the available pulse energies in our
system was imposed by multiple pulse instabilities
that tend to arise at high-pulse energies. This is a
consequence of the high-peak intensities present in
this laser and the saturation of the self amplitude
modulation effects. As the length of the laser cavity
is increased, the intracavity pulse energies and intensities are also increased to higher levels than in standard lasers.
A detailed understanding of the KLM mechanism and
the cavity operation is required for further improvement of the laser operation. We are currently optimizing the laser design in order to achieve shorter pulse
duration and higher output power while avoiding multiple pulsing.
1.14.2 Novel Saturable Absorber Materials
and Devices
Over the past several years, significant advances
have been made in generation of ultrashort optical
pulses from passively modelocked solid state lasers.
Kerr-lens modelocking is the most widely used modelocking technique, but it is generally not self-starting
and requires very precise resonator design and
52
53
alignment. For starting and stabilization of modelocking, a fast saturable absorber with low losses and
appropriate saturation intensity is required.
In recent years, there has been a large amount of
work done in application of semiconductor saturable
absorbers to laser modelocking. 52 Compared to other
approaches to laser modelocking, saturable absorbers have a number of advantages, including selfstarting operation, robustness and simplicity of laser
design, and a number of adjustable parameters
including operating wavelength, recovery time, and
saturation fluence which make it possible to tailor the
device parameters for a specific application. This
powerful technology has been applied for both modelocking and self-starting in a wide range of laser
materials in the 0.8, 1.0, 1.3, and 1.5 pm wavelengths regions.
Although this technology has significant advantages,
it also has the major disadvantage of requiring epitaxial growth (typically molecular or chemical beam
expitaxy) of semiconductor layers. Epitaxial materials
growth is complex and costly and thus is a barrier to
widespread commercial application of semiconductor
saturable absorbers.
Semiconductor doped color glasses, which have high
nonlinearities and rapid relaxation times, can potentially offer a simple and inexpensive alternative to
semiconductor saturable absorber mirrors. In earlier
studies, infrared color filter glass from Hoya (Hoya
IR76) has been demonstrated to generate self-starting 2.7 ps pulses from a Ti:AI20 3 laser."3
We have designed novel modelocking devices based
on commercial color filter glass for both saturable
absorber and self-starting KLM operation in a
Ti:A120 3 laser. Commercial semiconductor doped
color glasses from Schott (RG-850 and RG-780)
were attached to silica or sapphire substrates and
the structure was ground at an angle of 1 degree
resulting in a wedge of color glass on top of the substrate. The wedge-shaped design enabled us to
obtain thinner color glass than otherwise possible
(down to 20 pm) and to continuously vary the saturable absorber loss by changing the color glass inser-
U. Keller, K.J. Weingarten, F.X. Kartner, D. Kopf, B. Braun, I.D. Jung, R. Fluck, C. Honninger, N. Matuschek, and J. Aus der Au, "Semiconductor Saturable Absorber Mirrors (SESAM's) for Femrntosecond to Nanosecond Pulse Generation in Solid-State Lasers," IEEE J.
Select. Top. Quantum Electron. 2:435-53 (1996); S. Tsuda, W.H. Knox, ST. Cundiff, W.Y. Jan, and J.E. Cunningham, "Mode-Locking
Ultrafast Solid-State Lasers with Saturable Bragg Reflectors," IEEE J. Select. Top. Quantum Electron. 2: 454-64 (1996).
N. Sarukura, Y. Ishida, T Yanagawa, and H. Nakano, "All Solid State CW Passively Mode-locked Ti:Sapphire Laser Using a Colored
Glass Filter," Appl. Phys. Lett. 57: 229-30 (1990).
115
Chapter 1. Optics and Quantum Electronics
tion. The sapphire substrate made the structure more
robust than previous designs using thin free standing
glass and also served as an efficient heat sink,
reducing parasitic thermal effects.
We used a standard z-shaped cavity terminated with
an additional fold comprising two 10 cm radius of curvature mirrors. By placing the saturable absorber at
or near the focus of the additional fold, we obtained
self-starting saturable absorber modelocked operation with pulse durations of 1.9 ps. Varying the saturable absorber position relative to the focus enabled
us to vary the effective saturation cross-section.
Using a birefringent filter, tunable modelocked operation was achieved from 780 to 850 nm using RG-780
glass and from 840 to 870 nm using RG-850 glass.
Self-starting modelocked operation was also
achieved using the RG-850 glass saturable absorber
in conjunction with a resonator optimized for Kerr
lens modelocking (KLM) resulting in self-starting 52
fs transform limited pulses. The self-starting KLM
operation was robust and insensitive to cavity perturbations. An intracavity chopper was used to study the
modelocking self-starting dynamics by monitoring
both the average power and the second harmonic
(SHG) signal. The modelocking build up time was
measured to be 1.5 ms.
We believe that the spectrum, and therefore the ultimate pulse duration, is limited by the spectral filtering
combination of the mirror set on the long wavelength
side and the rapidly increasing absorption of the RG850 glass on the short wavelength side. Using thinner glass structures or color glass with lower semiconductor doping density would greatly increase
tunabilty to shorter wavelengths and support broader
spectra, thus leading to shorter pulses. We are also
currently working on incorporating the color glass
saturable absorber into a reflective geometry dielectric mirror device which will further simplify the laser
design.
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We believe that devices incorporating commercial or
experimental semiconductor doped color glasses can
potentially serve as simple and inexpensive devices
for both initiating KLM and modelocking solid-state
lasers.
1.14.3 Rapid Wavelength Tuning of Near-IR
Lasers
Rapid wavelength tuning of solid-state lasers is
important for a number of applications including optical ranging, optical communications, Raman spectroscopy, and fluorescence spectroscopy. Earlier
studies of frequency tunable sources used methods
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54
such as acousto-optic tuning and thermal tuning
to scan the wavelength range.
In the past year, we have extended our previous
research on the spectroscopy and modelocking of
chromium-doped forsterite (Cr4 +:Mg 2SiO 4 ) lasers to
develop a new rapidly tunable source in the near
infrared. Cr 4+:forsterite has a 200 nm cw tuning
range,5 6 making it suitable for rapidly and broadly
tunable laser. We implemented this laser using a
standard z-cavity configuration. The wavelength
selection, as well as spectral filtering, was accomplished by inserting four Brewster angled prisms and
a cavity end-mirror into one arm of the cavity. The
angular deflection of a light beam passing through
the prisms is wavelength dependent, and therefore a
narrow band of frequencies will be returned after
reflecting from the end mirror; more prisms give
stronger wavelength filtering. By tilting the end mirror,
we tuned the frequencies emitted from the laser. This
was done by placing the end-mirror upon a tilting galvanometer, which was driven with triangular voltage
waveforms at frequencies up to 1 kHz and angular
deflection amplitudes sufficient to cover the bandwidth of the laser. With this source, the tuning range
of 1200 to 1275 nm can be swept in less than 500 ps.
Scanning speeds are limited by cavity mode buildup
times and relaxation oscillation effects. Cr:forsterite
presents a broadband, relatively high power, and
simple rapidly tunable laser source in the 1.3 pm
wavelength region, important for optical communications.
K. Takada, "High-Resolution OFDR with incorporated Fiber-Optic Frequency Encoder," IEEE Photonics Technol. Lett. 4:1069-72
(1992)
U. Glombitza and E. Brinkmeyer, "Coherent Frequency-Domain Reflectometry for Characterization of Single-Mode Integrated Optical
Waveguides," J. Lightwave Technol. 11: 1377-84 (1993)
B. Golubovic, Study of Near-Infrared Pumped Solid-State Lasers and Applications, Ph.D. diss., Department of Electrical Engineering
and Computer Science, MIT, 1997.
RLE Progress Report Number 140
Chapter 1. Optics and Quantum Electronics
In optical frequency domain reflectometry (OFDR), a
cw laser is used as the input to an interferometer and
is swept over a wide range of frequencies. A delay
introduced into one arm of the interferometer causes
a difference between the frequencies of the light
which are returned from the two arms. This frequency difference can be detected by interfering the
two beams and can then be used to determine the
path-length difference between the arms of the interferometer. By utilizing a frequency-domain technique
to tune the laser, we demonstrated OFDR and imaging at resolutions similar to those achieved using
modelocked lasers and low-coherence interferometry. The frequency domain techniques can have the
advantage of a significant reduction in system complexity and still achieve high resolution since the distance resolution of the system is proportional to the
tuning bandwidth of the laser.
1.14.4 Optical Frequency Domain
Reflectometry Using Rapid
Wavelength Tuning
Optical ranging and tomographic techniques have
applications to various fields from engineering to
medicine. Optical frequency domain reflectometry
(OFDR) is based on interferometry and allows for
very high-resolution axial measurements compared
to optical time domain reflectometry (OTDR).
Another attractive feature of OFDR is that ranging
can be performed without the need for mechanical
scanning of the path length of the interferometer reference arm like in optical coherence tomography
(OCT),8 and it scans the wavelength of a narrowband source. Moreover, obtaining high single-transverse-mode power from a broadband light source
may present technical difficulties; a tunable narrowband source of equal brightness may be significantly
less complex.
OFDR uses a fixed reference path and continuously
sweeps a narrow-band optical source over a wide
span of frequencies. In this case, the wavelength of
the light detected at the interferometer output port is
a function of time. Ranging is performed by means of
a Michelson interferometer for which both arms are
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of fixed length, one terminated by a reference mirror,
and the other by the sample under investigation.
Introducing a relative temporal delay into this interferometer will then give rise to a difference in the wavelength of the light emitted from each arm. The
frequency of the beat at the output port of the interferometer is then used to determine the pathlength
difference and to localize backreflection sites in the
sample arm. Previous studies of the use of tunable
sources for OFDR were typically based on frequency-tunable semiconductor diode laser systems.
In our system we present the use of a rapidly tuned
cw Cr 4 :forsterite(Cr4:Mg 2SiO 4) laser 59 for OFDR
applications.
This system achieves high resolution imaging utilizing the high power and broad bandwidth potential of
solid-state lasers. A depth resolution of 15 pm was
demonstrated, with speeds as fast as 2000 longitudinal scans per second. The signal-to-noise ratio of
this system was measured to be 70 dB. Using other
tunable solid-state laser materials, one could perform
optical ranging and OCT with high resolution at other
wavelengths at which ultrashort-pulse generation is
problematic. The ultimate extension of these techniques using narrow-linewidth tunable diode laser
sources would afford even higher scanning speeds
and greater ease of use.
1.14.5 Publications
Bilinsky, I.P.,
B.E. Bouma, and J.G. Fujimoto. "Selfstarting KLM Ti:AI203 Laser Using Semiconductor Doped Glass Structures." Paper presented at
the 1998 Conference on Lasers And ElectroOptics, San Francisco, California, May 2-8,
1998
Cho, S.H., B.E. Bouma, E.P. Ippen, and J.G. Fujimoto. "A 15 MHz High Peak Power KLM Ti:AI20 3
Laser using Multiple Pass Long Cavity." Paper
presented at the 1998 Conference on Lasers
And Electro-Optics, San Francisco, California,
May 2-8, 1998.
Golubovic, G. B.E. Bouma, G.J. Tearney, and J.G.
Fujimoto. "Optical Frequency-Domain Reflectometry using Rapid Wavelength Tuning of a
S.D. Personick, "Photon Probe Optical-fiber Time-domain Reflectometer," Bell Syst. Tech. J. 56: 355-66 (1977); D. Uttam and B.
Culshaw, "Precision Time Domain Reflectometry in Optical Fiber Systems using a Frequency Modulated Continuous Wave Ranging
Technique," J. Lightwave Technol. 3: 971-977 (1985).
D. Huang, E.A. Swanson. C.P. Lin, J.S. Schuman, W.G. Stinson, W. Chang, M.R. Hee. T. Flotte, K. Gregory, C.A. Puliafito, and J.G.
Fujimoto, "Optical Coherence Tomography," Sci. 254: 1178-81 (1991).
B. Golubovic, B.E Bouma, I.P. Bilinsky, J.G. Fujimoto, and V.P. Mikhailov, "Thin Crystal, Room-temperature Cr 4 :Forsterite Laser using
Near-infrared Pumping," Opt. Lett. 21: 1993-95 (1996).
Chapter 1. Optics and Quantum Electronics
Cr4+:Forsterite
Laser." Opt. Lett. 22: 1704-06
(1997).
1.15 Laser Medicine and Medical Imaging
Sponsors
National Institutes of Health
Grant 9RO1 EY11289-12
Grant 1R01 CA75289-01
U.S. Air Force - Office of Scientific Research
Contract F49620-95-1-0221
U.S. Navy - Office of Naval Research/MFEL
Contract N00014-94-1-0717
Contract N000014-97-1-1066
Project Staff
Stephen A. Boppart, Dr. Brett E. Bouma, Dr. Mark E.
1
Brezinski,6 ° Michael R. Hee. Dr. Jurgen Herrmann,
Constantinos Pitris, Guillermo J. Tearney, Professor
62
James G. Fujimoto, Eric A. Swanson, Dr. Carmen
64
3
Puliafito, Dr. Joel Schuman
1.15.1 Optical Coherence Tomography
Technology
Optical coherence tomography (OCT) is a new imaging technology which was developed by our group in
1991. 6 OCT is analogous to ultrasound or radar
imaging except that it uses light instead of sound to
perform high resolution cross sectional imaging of
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biological specimens and materials. It allows in vitro
or in vivo imaging of tissue in virtually any organ
accessible via catheter or endoscope, generating
two-dimensional tomographic images of tissue
microstructure or three-dimensional reconstructed
volumes.
OCT was first used to image optically transparent
66
structures such as the anterior eye and retina. Preliminary clinical investigations suggest that OCT is a
promising technology for the detection and management of a variety of retinal diseases including glaucoma and macular edema. 7 Recent advances have
led to the application of this modality to non-transparent tissue.68 Although the imaging depth of OCT is
limited by the light scattering and attenuation properties of tissue, penetration of 2-3 mm can be achieved
even in heavily calcified samples. The resolution of
OCT is on the micron scale, up to two orders of magnitude higher than conventional ultrasound. In
essence, imaging is being performed over the distance of a conventional biopsy and near the resolution of histopathology, hence the description of OCT
as an "optical biopsy." Previous studies with OCT
have included the identification of pathology in the
cardiovascular system, gastrointestinal tract, and
skin, in addition to studies of normal urinary tract,
69
nervous system, and reproductive tract. A small
catheter-endoscope system has also been developed which allowed high resolution imaging to be
70
performed in vivo in a rabbit model.
Research Affiliate, Cardiac Unit, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts.
Visiting Scientist, University of Erlangen, Germany.
MIT Lincoln Laboratory. Lexington, Massachusetts.
Chairman. Department of Ophthalmology, New England Eye Center, Tufts University Medical School and Director, Boston, Massachusetts.
Director, Glaucoma Services, New England Eye Center, Tufts New England Medical Center, Boston, Massachusetts.
D. Huang, E.A. Swanson, C.P. Lin, J.S. Schuman. W.G. Stinson, W. Chang, M.R. Hee, T. Flotte, K. Gregory, C.A. Puliafito, J.G. Fujimoto, "Optical Coherence Tomography," Sci. 254: 1178-81 (1991).
E.A. Swanson, D. Huang, C.P. Lin, J.S. Schuman, C.A. Puliafito, and J.G. Fujimoto, "Optical Coherence TomogM.R. Hee, J.A. Izatt,
raphy of the Human Retina," Arch. Ophthalmol. 113: 325-32 (1995); E.A. Swanson, J.A. Izatt, M.R. Hee, D. Huang, C.P. Lin, J.S.
Schuman, C.A. Puliafito, and J.G. Fujimoto, "InVivo Retinal Imaging by Optical Coherence Tomography," Opt. Lett. 18: 1864-66
(1993).
C.A. Puliafito. M.R. Hee, C.P. Lin, E. Reichel, J.S. Schuman, J.S. Duker, J.A. Izatt, E.A. Swanson, and J.G. Fujimoto, "Imaging of Macular Diseases with Optical Coherence Tomography," Ophthalmol. 102: 217-29 (1995).
C.A. Puliafito, M.R. Hee, J.S. Schumann, and J.G. Fujimoto, "Optical Coherence Tomography of Ocular Diseases," (Thorofare, New
Jersey: SLACK Incorporated, 1995); J.G. Fujimoto, M.E. Brezinski, G.J. Tearney, S.A. Boppart, B.E. Bouma, M.R. Hee, J.F. Southern,
and E.A. Swanson, "Optical Biopsy and Imaging Using Optical Coherence Tomography," Nature Med. 1: 970-72 (1995); J. Schmitt, M.
Yadlowsky, and R. Bonner, "Subsurface Imaging of Living Skin with Optical Coherence Microscopy," Dermatol. 191: 93-98 (1995).
M.E. Brezinski, G.J. Tearney, B.E. Boumna, J.A. Izatt, M.R. Hee, E.A. Swanson, J.F. Southern, and J.G. Fujimoto, "Optical Coherence
Tomography for Optical Biopsy. Properties and Demonstration of Vascular Pathology," Circulat. 93(6): 1206-13 (1996).
Y. Pan, E. Lankenau, J. WIezel, R. Birngruber, and R. Engelhardt, "Optical Coherence-gated Imaging of Biological Tissues," Sel. TopM.D. Kulkarni, H.W. Wang, K. Kobayashi, and M.V. Sivak, "Optical Coherence
ics Quantum Electron. 2:1029-34 (1996); J.A. Izatt,
Tomography and Microscopy in Gastrointestinal Tissues," Sel. Topics Quantum Electron. 2:1017-28 (1996).
RLE Progress Report Number 140
Chapter 1. Optics and Quantum Electronics
OCT is based on the detection of infrared light waves
that are back-scattered or reflected from different
layers and structures within the tissue. Utilizing a
Michelson interferometer, the beam leaving the optical light source is split into two parts, a reference and
a sample beam. The reference beam is reflected off
of a mirror at a known distance and returns to the
detector. The sample beam reflects off of different
layers within the tissue. Light returning from the sample and reference arms recombines and if the two
light beams have traveled the same distance (optical
pathlength), the two beams will interfere. Low coherence can be used to localize backreflection sites and
provide the desired high axial resolution. OCT measures the intensity of the interference obtained from
different points within the tissue by moving the mirror
in the reference arm which changes the distance light
travels in that arm. Two or three dimensional images
are produced by scanning the beam across the sample and recording the optical backscattering versus
depth at different transverse positions. The resulting
data is a two or three dimensional representation of
the optical backscattering of the sample on a micron
scale.
With previous OCT systems, the optical delay in the
reference arm was varied with either a linearly translating galvanometer or by stretching an optical fiber
with a piezoelectric crystal./1 However, commercial
galvanometers do not generate sufficient mechanical
translation rates to allow imaging in real time. Piezoelectric fiber stretchers allow rapid scanning, but they
suffer from high power requirements, nonlinear fringe
modulation due to hysteresis, uncompensated polarization dispersion matches, and poor temperature
stability. For these reasons, we designed a highspeed optical delay line using phase control techniques originally developed for femtosecond pulse
shaping. 72 This device can be constructed with com-
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mon optical components, has modest power requirements, is repeatable, and is temperature stable.7"
The phase control optical delay line contained a lensgrating pair to Fourier transform the temporal profile
of the low-coherence (broad-spectrum) light in the
reference arm. A mirror, mounted to a galvanometer,
placed at the Fourier plane, allowed angular tilt to be
mapped to group delay.74 The group delay was varied by rapidly changing the angle of the mirror
mounted to the galvanometer, allowing the acquisition of 2000 axial scans per second, or 4 to 8 frames
per second, with a total optical path length delay of
~3 mm. This method, in conjunction with a fiber optic
catheter/endoscope was used to perform in vivo
OCT imaging for the first time.7
The delivery of optical radiation for medical diagnostic and imaging applications has motivated several
unique optical system designs. Forward-imaging
devices overcome a limitation of side-imaging
devices; permitting data to be collected before introducing the device into tissue. This concept also permits the image-guided placement of the device in
surgery and for monitoring interventional procedures
such as tissue incision, resection, and laser surgery.
To perform forward-imaging of a single-mode beam,
a lens, fiber, or combination thereof may be translated with motors, piezoelectric cantilevers, or electrostatic/magnetic techniques.
A second design permits variable magnification by
scanning a cleaved fiber tip in the focal plane of the
first lens in a telescope. The beam is collimated and
then focused by a second lens which effectively
relay-images the fiber tip onto the object or specimen. The advantages of this design include low
mass on the piezoelectric cantilever permitting higher
translation velocities and interchangeable magnification via different lenses. 76 In contrast to the hand-held
V.M. Gelikonov, G.V. Gelikonov, R.V. Kuranov, K.I. Pravdenko, A.M. Sergeev, F.I. Feldshtein, Y. I. Khanin, D.V. Shabanov, N.D. Gladkova, N.K. Nikulin, G.A. Petrova, and V.V. Pochinko, "Coherent Optical Tomography of Microscopic Inhomogeneities in Biological Tissues," JETP Lett. 61(2): 158-62 (1995); G.J. Tearney, B.E. Bouma, S.A. Boppart, B. Golubovic, E.A. Swanson, and J.G. Fujimoto,
"Rapid Acquisition of In Vivo Biological images by use of Optical Coherence Tomography," Opt. Lett. 21(17): 1408-10 (1996).
K.F. Kwong, D. Yankelevich, K.C. Chu, J.P. Heritage, and A. Dienes, "400-Hz Mechanical Scanning Optical Delay Line," Opt. Lett.
18(7): 558-60 (1993); J.A. Izatt, M.D. Kulkarni, H.W. Wang, K. Kobayashi, and M.V. Sivak, "Optical Coherence Tomography and
Microscopy in Gastrointestinal Tissues," Sel. Topics Quantum Electron. 2:1017-28 (1996).
G.J. Tearney, B.E. Bouma, and J.G. Fujimoto, "High Speed, Phase- and Group-delay Scanning with a Grating-based Phase Control
Delay Line," Opt. Lett. 22: 1811-13 (1997).
R.P. Salathe, H. Gilgen, and G. Bodmer, "Coupled-mode Propagation in Multicore Fibers Characterized by Optical Low-coherence
Reflectometry," Opt. Lett. 21(13): 1006 (1996).
G.J. Tearney, M.E. Brezinski, B.E. Bouma, and S.A. Boppart, C. Pitris, J.F. Southern, and J.G. Fujimoto, "In Vivo Endoscopic Optical
Biopsy with Optical Coherence Tomography," Sci. 276: 2037-39 (1997).
S.A. Boppart, B.E. Bouma, C. Pitris, G.J. Tearney, and J.G. Fujimoto, "Forward-scanning Instruments for Optical Coherence Tomographic Imaging," Opt. Lett. 22: 1618-20 (1997).
Chapter 1. Optics and Quantum Electronics
probe for open-field surgical procedures, rigid laparoscopes permit visualization of tissue at distant (10-50
cm) internal sites while maintaining a small instrument diameter for insertion through small incisions
during minimally invasive surgical procedures. A single rod lens or a series of relay lenses (Hopkins-type)
can be used to image. The laparoscope also permits simultaneous en face OCT imaging of the area.
A rigid OCT laparoscope was constructed using a
2.68 mm diameter, 19.5 cm long, rod lens with a
nominal pitch length of 3/2 for visible wavelengths. To
demonstrate the imaging capability of the hand-held
probe design, in vitro images of human ovary and
lung were acquired with 12 pm axial and 31 pm
transverse resolution. The external surface imaging
orientation for the ovary is similar to that encountered
during open-field surgical procedures. In summary,
we have presented hand-held probe and rigid laparoscope forward-imaging optical instruments for optical
coherence tomographic imaging. These two devices
represent different approaches for OCT imaging and
can be powerful diagnostic tools for medical and surgical applications. In future applications, OCT imaging might also be integrated with laser-based surgery
to permit the simultaneous visualization and surgical
incision of tissues on a micron scale.
1.15.2 Optical Biopsy Using Optical
Coherence Tomography
A technology capable of performing optical biopsy
should prove to be a powerful diagnostic modality in
clinical medicine. Optical biopsy is defined here as
imaging tissue microstructure at or near the level of
histopathology without the need for tissue excision.
At least three clinical scenarios exist in which optical
biopsy will likely have a considerable impact on
patient management. The first is in situations in
which sampling errors severely restrict the effective-
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ness of excisional biopsy, such as the high failure
rates associated with blind biopsies used to screen
the premalignant conditions of ulcerative colitis or
Barrett's esophagus. 78 A need also exists for optical
biopsy when conventional excisional biopsy is potentially hazardous. Examples of vulnerable regions
include the central nervous system, the vascular system, and articular cartilage. Finally, the ability to
image at the cellular level could improve the effectiveness of many surgical and microsurgical procedures including coronary atherectomy, transurethral
prostatectomies, and microvascular repair. 9
Myocardial infarction, better known as a heart attack,
remains one of the most commonly diagnosed problems in hospitalized patients in the United States.
Most heart attacks are caused by the rupture of an
atherosclerotic plaque, which results in thrombus formation and subsequent coronary artery occlusion.
Plaques exhibiting large lipid cores held by weak
fibrous caps are at the greatest risk of rupture. Currently, imaging modalities such as angioscopy, ultrasound, and MRI are not able to identify these highrisk plaques. Angioscopy is limited because only the
surface of the artery can be viewed in the absence of
blood. Ultrasound and MRI are both limited by low
resolution and dynamic range. In addition, MRI is an
expensive and complicated diagnostic tool. OCT's
high resolution enabled a series of in vitro experiments to examine arterial structure including intimal
wall thickness, lipid content, and even heavily calcified plaques.8" Following the development of a catheter-based OCT system, an in vitro study comparing
the resolution of OCT and intravascular ultrasound
was performed. The images obtained using OCT
yielded superior structural detail in all plaques examined.8 OCT's fiber optic design has the ability to be
delivered through intravascular catheters, making
clinical diagnosis and surgical guidance practical
T.H. Tomkinson. J.L. Bently, M.K. Crawford, C.J. Harkrider, D.T. Moore, and J.L. Rouke, "Rigid Endoscopic Relay Systems: A Comparative Study," Appl. Opt. 35(34): 6674-83 (1996).
R.W. Phillips and R.K.H. Wong, "Barret's Esophagus. Natural History. Incidence. Etiology, and Complications." Gastroenterol. Clin. N.
Am. 20: 791 (1991).
G.J. Tearney, M.E. Brezinski, J.F. Southern, B.E. Bouma, S.A. Boppart, and J.G. Fujimoto, "Optical Biopsy in Human Urologic Tissue
Using Optical Coherence Tomography" J. Urol. 157: 1915-19 (1997); M.E. Brezinski, G.J. Tearney, S.A. Boppart, E. A. Swanson, J.F.
Southern, and J.G. Fujimoto, "Optical Biopsy with Optical Coherence Tomography: Feasibility for Surgical Diagnostics," J. Surgical
Res. 71(1): 32-40 (1997).
M.E. Brezinski, G.J. Tearney, B.E. Bouma, S.A Boppart, M.R. Hee, E.A. Swanson, J.F. Southern, J.G. Fujimoto, "Imaging of Coronary
Artery Microstructure (In Vitro) with Optical Coherence Tomography," Am. J. Cardiol. 77: (1996).
G.J. Tearney, M.E. Brezinski, S.A. Boppart, B.E. Bouma, N. Weissman, J.F. Southern, E.A. Swanson, and J.G. Fujimoto, "Catheterbased Optical Imaging of a Human Coronary Artery," Circulat. 94: 3013 (1996); M.E. Brezinski, G.J. Tearney, N.J. Weissman, S.A.
Boppart, B.E. Bouma, M.R. Hee. A.E. Weyman, E.A. Swanson, J.F. Southern. and J.G. Fujimoto, "Assessing Atherosclerotic Plaque
Morphology: Comparison of Optical Coherence Tomography and High Frequency Intravascular Ultrasound," Brit. Heart J. 77: 397-403
(1997).
120 RLE Progress Report Number 140
Chapter 1. Optics and Quantum Electronics
applications of this technology. The ability to detect
small atherosclerotic plaques as well as existing fissures in the vessel long before they rupture has obvious benefits in heart attack prevention.
Cancer is second only to heart disease as a leading
cause of mortality in the industrialized world." Effective screening and detection of early neoplastic
changes are important in improving a patient's prognosis, since once metastatic. treatment is difficult
while cure, for the most part, is not possible." Many
existing diagnostic modalities, including x-ray imaging, ultrasound, and endoscopy do not have sufficient
resolution to detect changes associated with early
neoplasia. Excisional biopsy, which is the gold standard for diagnosis, is based mostly on the identification of structural and cellular changes associated
with the disease. Excision of specimens, however, is
sometimes not possible, has limited area coverage,
and can lead to unacceptable false negative rates
because of sampling errors.84 Interventional decisions require accurate diagnosis and positive identification of the tumor margins which are not easily and
readily quantifiable due to the limitations associated
with conventional biopsies.
A diagnostic imaging technology that can perform
real-time, high resolution tomographic imaging of the
architectural morphology of tissue in situ could
improve the diagnosis of early neoplasia and thus
reduce morbidity and mortality. Optical coherence
tomography cold fill this void. OCT imaging was performed in vitro on a series of human tissues of varying degrees of neoplastic infiltration, including colon,
cervix, uterus and lung Microstructural and epithelial
changes associated with neoplasia, such as dilated
and distorted glands, were readily imaged and favorably matched to histopathology. We were also able to
investigate the structure and organization of the epithelium and assess where it was destroyed and the
integrity of the basal membrane was compromised.
In more advanced stages of cancer, neo-vascularization was also evident.
Although the penetration of OCT imaging is limited to
a few millimeters, it is sufficient to image epithelial
cell layers in most organ systems and detect
changes associated with early neoplasia. OCT features. described earlier, coupled with the recent
experimental results suggest that OCT could provide
an effective tool for the diagnosis and assessment of
neoplastic changes of tissue.R
1.15.3 In-Vivo Imaging with Optical
Coherence Tomography
A major step in promoting OCT as a clinically viable
and diagnostically useful imaging modality, is investigating and assessing its capabilities in vivo. To do so,
we have demonstrated in vivo endoscope-based
OCT imaging of the gastrointestinal, respiratory and
cardiovascular tracts in an animal model with an axial
resolution of 10 mm at four to eight frames per second. The optical delay line and catheter/endoscope,
both described earlier, were used to image normal,
12-week-old New Zealand White rabbits.
OCT images of the in vivo rabbit esophagus allowed
visualization of all layers of the esophageal wall. For
example, the innermost layer, the mucosa, was
readily distinguished owing to its low reflectivity compared with the submucosa. Vascular structures were
also identified within the wall. These high-resolution
images demonstrate the capability of OCT to both
resolve micro-structural detail and image the entire
rabbit esophagus to the serosa. In vivo OCT images
of the rabbit trachea permitted differentiation of the
pseudostratified epithelium, mucosa, and surrounding hyaline cartilage. Because most neoplasms of
the esophagus and respiratory tract originate in the
epithelium, the ability of OCT to precisely identify the
mucosa could have important clinical implications.
In vivo OCT imaging (Figure 17) of a living rabbit's
aorta was performed via a small, catheter-based system with a 10 pm resolution. OCT's intravascular
imaging represented a ten fold improvement in resolution over "state of the art" intravascular ultrasound
(IVUS) transducers. High contrast was noted
82
S.L. Parker. T. Tong. S. Bolden, and P.A. Wingo, "Cancer Statistics 1997," CA 47(1): 5-27 (1997).
83
R.T. Osteen, ed., Cancer Manual, 8th ed. (Boston: American Cancer Society, 1990).
84
K.W. Wang and E.P. DiMagno, "Endoscopic Ultrasonography: High Technology and Cost Containment," Gastroenterol. 105(1): 283-86
(1993).
85
G.J. Tearney, S.A. Boppart. B.E. Bouma, M.E. Brezinski, N.J. Weissman, J.F. Southern, and J.G. Fujimoto. "Scanning Single-Mode
Fiber Optic Catheter-Endoscope for Optical Coherence Tomography," Opt. Lett. 21(7): 543-45 (1996): M.E. Brezinski, G.J. Tearney,
B.E. Bouma, J.A. Izatt, M.R. Hee, E.A Swanson, J.F. Southern, and J.G. Fujimoto. "Optical Coherence Tomography for Optical
Biopsy. Properties and Demonstration of Vascular Pathology," Circulat. 93(6): 1206-13 (1996).
Chapter 1. Optics and Quantum Electronics
between the media and the surrounding supportive
tissue. In addition, fine structural detail was noted
within the surrounding supportive tissue. This work,
taken together with previous studies examining in
vitro atherosclerotic plaque, strongly suggest a role
for OCT in high-resolution intraarterial assessments
of coronary atherosclerosis. An observation of particular importance in this study was the attenuation
which resulted from the presence of blood. No significant structural information was obtained unless
simultaneous injections of saline, at a relatively low
pressure and small volumes, were performed. This
likely results from the large numbers of scattering
86
particles (i.e., RBCs) present within blood. If saline
injections are required for in vivo patient imaging, clinicians will need to weigh the increased resolution
and low cost against the likely increase in procedure
time. In this work, imaging of atherosclerotic samples
was not performed. Future technology development
will focus on increasing data acquisition rates and
integrating the system with low cost light sources.
Figure 17. Rabbit aorta images after saline injection. Initiation of injection (A), maximal distention (B) and
relaxation (C). Also visible are the media (m) and adventia (a) and a vein adjacent to the aorta (v).
1.15.4 Guiding Microsurgical Intervention
with Optical Coherence Tomography
vitro surgical tissue specimens have shown potential
for surgical diagnostics within the surgical operating
87
suite.
Optical instruments have become well-integrated into
the surgical setting to visualize tissue and improve
patient outcome. Surgical microscopes and loupes
magnify tissue to prevent iatrogenic injury and to
guide delicate surgical techniques. Although the
repair of sensitive structures are performed with the
aid of surgical microscopes to magnify the surgical
field, surgeons have been limited to the en face view
that they provide. OCT provides a technique capable
of subsurface, three-dimensional, micron-scale imaging in real-time. This permits the intraoperative monitoring of microsurgical procedures, offering
immediate feedback to the surgeon. Studies of in
OCT is an optical imaging technology ideally suited
for rapidly acquiring micron-scale resolution two- and
three-dimensional images of biological tissue, such
as small vessels and nerves. The OCT technology
can be readily integrated with medical and surgical
optical instruments. OCT imaging has been performed with a radial imaging flexible catheter/endoscope for accessing tortuous body lumens such as
88
the gastrointestinal tract and vascular system. A
hand-held surgical probe has been developed for
open-field surgical imaging and an OCT laparoscope
has been constructed for internal imaging during
86
87
88
J.M. Steinke and A.P. Shepherd, "Comparison of Mie Theory and the Light Scattering of Red Blood Cells," Appl. Opt. 27: 4027-33
(1988).
M.E. Brezinski, G.J. Tearney, S.A. Boppart, E.A. Swanson, J.F. Southern, and J.G. Fujimoto, "Optical Biopsy with Optical Coherence
Tomography, Feasibility for Surgical Diagnostics," J. Surg. Res. 71: 32-40 (1997).
G.J. Tearney, S.A. Boppart, B.E. Bouma, M.E. Brezinski, N.J. Weissman, J.F. Southern, and J.G. Fujimoto, "Scanning Single-mode
Fiber Optic Catheter-endoscope for Optical Coherence Tomography," Opt Lett. 21: 543-45 (1996).
122 RLE Progress Report Number 140
Chapter 1. Optics and Quantum Electronics
minimally invasive surgical procedures. 89 OCT can
function as a type of "optical biopsy," permitting the
imaging of in vivo biological morphology without the
need to excisionally remove and process a tissue
specimen as in conventional histology.
Microsurgical procedures involve the re-anastomosis
or re-attachment of severed nerves and vessels.
These delicate operations are necessary to restore
nerve function or vascular perfusion on the extremities or face following traumatic injury. To guide microsurgical interventions, the OCT technology was
integrated with a stereo surgical microscope. The
microscope enabled the location of the OCT imaging
beam to be observed with high precision for real-time
imaging during microsurgical procedures. 90 Arterial
lumen obstruction is a leading cause of failure following microsurgical procedures. Cross-sectional OCT
imaging enables immediate feedback to the operator
and was used to locate intraluminal obstructions and
potential thrombus-forming sites following the in vitro
anastomosis of a severed artery. Identifying potential
obstructions intraoperatively may improve the outcome of surgical procedures. In particular, identifying
possible thromobogenic sites may reduce the incidence of late vascular occlusion after the injury site
has been closed and bandaged.
OCT has also been used to image peripheral nerve
fascicles in two and three dimensions. A peripheral
nerve is composed of several smaller fascicle bundles. During surgical repair, corresponding fascicles
from severed ends must be appropriately connected
to effectively restore distal function. The three-dimensional imaging capabilities of OCT have been applied
to tracking individual nerve fascicles along the axis of
peripheral nerves. A 3D image with a segmented fascicle is shown in Figure 18. Also shown in this figure
is a bifurcation of one fascicle which was not evident
from 2D images. The use of OCT to intraoperatively
assess microsurgical repair and to avoid subsurface
vessels and nerves prior to incision offers promise for
reducing patient morbidity and mortality following
surgical procedures.
Figure 18. Three-dimensional OCT demonstrating
individual fascicles within a human peripheral nerve.
A bifurcation (b) observed in 3D was not apparent
from 2D images.
1.15.5 Ophthalmic Imaging and Diagnosis
with Optical Coherence Tomography
Strong collaborative efforts have led to the development and clinical application of an ophthalmic imaging OCT system. Working with MIT Lincoln Laboratory, a compact OCT system utilizing a fiber-coupled super-luminescent diode light source operating
at 840 nm was integrated with a clinical ophthalmic
slit-lamp biomicroscope. The biomicroscope permitted the operator to simultaneously visualize a
patient's retina while viewing the position of the OCT
imaging beam on the retina via a coincident visible
aiming beam. The fully computer-controlled OCT
system allowed arbitrary scan patterns on the retina
or anterior eye structures. 91 Automated image processing routines have been developed to determine
retinal thickness and produce multi-color topographic
maps for rapid clinical assessment of ocular pathologies.
89
S.A. Boppart, B.E. Bouma, C. Pitris, G.J. Tearney, M.E. Brezinski, and J.G. Fujimoto, "Forward-imaging Instruments for Optical Coherence Tomographic Imaging," Opt. Lett. 22:1618-20 (1997).
90
S.A. Boppart, B.E. Bouma, C. Pitris, G.J. Tearney, J.F. Southern, M.E. Brezinski, and J.G. Fujimoto, "Three-dimensional Optical
Coherence Tomography for Microsurgical Diagnostics," Radiol., forthcoming.
M.R. Hee, J.A. Izatt, E.A. Swanson, D. Huang, J.S. Schuman, C.P. Lin, C.A. Puliafito, and J.G. Fujimoto, "Optical Coherence Tomography of the Human Retina," Arch. Ophthalmol. 113: 325-32 (1995).
91
123
Chapter 1. Optics and Quantum Electronics
To date, ophthalmology has been the major clinical
application for OCT In collaboration with the New
England Eye Center of Tufts University School of
Medicine, over 5000 patients with a variety of macular diseases and diseases of the optic nerve head
have been assessed and followed. 92 The technology
has been transferred to industry and was introduced
for clinical and research use in 1996 by Humphrey
Instruments, a Division of Carl Zeiss, Inc. Approximately 100 units have been fielded to date. In many
cases, OCT serves as the definitive diagnostic tool
because it provides a cross-sectional image of retinal
structure in addition to the surface features seen on
fundoscopic exam. The cross-sectional view of OCT
has been effective in the diagnosis and monitoring of
macular holes, macular edema, retinal detachments, 93 macular degeneration, 94 and glaucoma.
The formation of macular holes is a retinal disease
which often progresses to complete loss of central
vision. OCT is useful for identifying early macular
hole formation, staging macular hole progression,
and for determining when surgical intervention is
necessary.95 Early surgical intervention can often
prevent or correct vision loss. Macular edema is a
disease where intraretinal fluid accumulation produces swelling and injury to the retina. The development of macular edema is a major treatable cause of
vision loss in patient with diabetes. OCT is a more
sensitive and objective indicator of retinal thickening
due to macular edema than either slit-lamp biomicroscopy or fluorescein angiography. OCT can also
quantitatively follow the resolution of edema following
therapy. A topographic display protocol of macular
thickness has been developed. The use of a standardized protocol means that OCT may have a significant impact on public health as a screening tool
for the development of retinopathy in diabetic
patients.
92
93
94
95
96
97
Recent efforts in the clinical application of OCT in
ophthalmology have focused on the early diagnosis
of glaucoma. Glaucoma is the third leading cause of
blindness in the United States Current diagnostic
techniques such as direct and indirect ophthalmoscopy and evaluation of the cup-to-disc ratio are subjective. Measuring peripheral field loss in glaucoma
patients provides quantitative parameters; however,
up to 50% of the retinal nerve fiber layer may be lost
prior to any noticeable symptoms. High-resolution
cross-sectional OCT images of the retina permit
reproducible nerve fiber layer thickness measurements within 10 to 20 microns." Nerve fiber layer
thickness measured by OCT correlates well with the
functional status of the optic nerve and the visual
field loss determined by traditional examination.
Novel computer algorithms have been implemented
to further improve the accuracy and monitoring of
changes in nerve fiber layer thickness.9 These
results indicate that OCT is potentially able to detect
the early onset of glaucoma before irreversible vision
loss occurs.
1.15.6 Subcellular Optical Coherence
Tomography Imaging: Implications
for the Early Detection of Neoplasms
OCT is an optical imaging technology capable of
subcellular resolutions which may ultimately have a
role in the early diagnosis of malignancies. Neoplasias are most responsive to medical intervention at
early stages, prior to undergoing metastasis. When
these disorders arise from known premalignant
states, and if a detection method exists, the high risk
population can be screened to reduce patient morbidity and mortality. Organs where these premalignant conditions occur at relatively high frequency
include the esophagus, uterus, bladder, colon, and
C.A. Puliafito, M.R. Hee, C.P. Lin, E. Reichel. J.S. Schuman, J.S. Duker, J.A Izatt, E.A. Swanson. and J.G. Fujimoto, "Imaging of
Macular Diseases with Optical Coherence Tomography," Ophthalmol. 102: 217-29 (1995).
M.R. Hee, C.A. Puliafito, C. Wong, E. Reichel, J.S. Duker. J.S. Schumrnan, E.A Swanson. and J.G. Fujimoto, "Optical Coherence
Tomography of Central Serous Chorioretinopathy," Am. J. Ophthalmol. 120: 65-74 (1995).
M.R. Hee, C.R. Baumal. C.A. Puliafito. J.S. Duker, E. Reichel, J.R. Wilkins. J.G. Coker. J.S. Schuman, E.A. Swanson, and J.G. Fujimoto, "Optical Coherence Tomography of Age-Related Macular Degeneration and Choroidal Neovascularization,' Ophthalmol. 103:
1260-70 (1996).
M.R. Hee. C.A. Puliafito, C. Wong, J.S. Duker, E. Reichel, J.S. Schuman. E.A. Swanson, and J.G. Fujimoto," Optical Coherence
Tomography of Macular Holes," Opthalmol. 102: 748-56 (1995).
J.S. Schuman, T. Pedut-Kloizman, E. Hertzmark. M.R. Hee, J.R. Wilkins, J.G. Coker. C.A. Puliafito. J.G. Fujimoto, and E.A. Swanson,
"Reproducibility of Nerve Fiber Layer Thickness Measurements using Optical Coherence Tomography." Ophtha!mol. 103: 1889-98
(1996).
D. Huang, M.R. Hee, L Pieroth, T. Pedut-Kloizman. J G. Coker, J.R. Wilkins. J.G. Fujimoto, E A. Swanson, C.A. Puliafito, and J.S.
Schuman, "A New Algorithm for Detecting Glaucomatous Nerve Fiber Layer Loss using Optical Coherence Tomography," Submitted to
Ophthalmol.
124 RLE Progress Report Number 140
Chapter 1. Optics and Quantum Electronics
stomach. The ability to image these organs in realtime at subcellular resolutions could represent a
powerful tool for the early identification of neoplasms.
New technologies which have been pursued, with
limited success, for the high-resolution assessment
of subcellular structure include magnetic resonance
imaging (MRI), x-ray microscopy, and high frequency
ultrasound. These techniques, however, suffer from
long acquisition times, potentially hazardous ionizing
radiation, and direct contact with tissue, respectively.
Recent advances in high-speed, in vivo, laser-scanning confocal microscopy have provided researchers
with a means of imaging cellular morphology in
skin. 98 However, confocal microscopy has a penetration depth limited to a few hundred microns and cannot be readily performed through an endoscope.
OCT combines the high resolutions of most optical
techniques with an ability to reject multiply scattered
photons and hence image at cellular resolutions up
to millimeters deep in non-transparent tissue. Penetration limitations of other optical technologies are
overcome by the use of near-infrared wavelengths
which are absorbed and scattered less than shorter,
visible wavelengths. Because OCT relies on variations in index of refraction and optical scattering for
image contrast, no exogenous fluorophores are necessary for imaging which can limit the viability of living cells. OCT has been demonstrated as a useful
research microscopy technique for developmental
biology. In vivo developing embryos can be followed
to observe the morphological expression of genes
and genetic abnormalities.99 High-speed OCT imaging has permitted the in vivo assessment of functional cardiovascular parameters in developing
specimens. 00
Although previous studies have demonstrated in vivo
OCT imaging of tissue morphology, most have
imaged tissue at resolutions of -10-15 pm, which
does not allow differentiation of cellular structure.
The Xenopus laevis (African frog) tadpole, a common
developmental biology animal model, was used to
demonstrate the feasibility of OCT for in vivo subcellular imaging. A broad spectral bandwidth Kerr-lens
modelocked Cr4:forsterite laser enabled resolution
as high as 5 pm. OCT was able to identify the mitotic
activity, the nuclear-to-cytoplasmic ratio, and the
migration of developing cells for extended periods of
time. 101 OCT images of cell structures were highly
correlated with corresponding histology. A cross-sectional OCT image of actively dividing cells is shown
in Figure 19. Individual cells, cell membranes, and
cell nuclei are readily observed deep within the scattering tissue of a non-transparent tadpole. Melaninladen neural crest cells are also observed along with
the large hemisphere of the neural tube (brain).
Three-dimensional neural crest cell migration has
been followed in these specimens. These results
have implications for OCT imaging of in vivo cellular
morphology which, on extension to humans, could
represent a powerful tool for the early evaluation of
neoplastic changes.
Figure 19. Subcellular OCT imaging within a
developing Xenopus tadpole. Mitotically active cells
(c) and migrating neural crest cells (nc) are observed
in vivo next to the neural tube (nt) or brain of the
tadpole.
98
M. Rajadhyaksha, M. Grossman, D. Esterowitz, R.H. Webb, and R.R. Anderson, "In vivo Confocal Scanning Laser Microscopy of
Human Skin: Melanin Provides Strong Contrast," J. Invest. Dermatol. 104: 946-52 (1995).
99 S.A. Boppart, M.E. Brezinski, B.E. Bouma, G.J. Tearney, and J.G. Fujimoto, "Investigation of Developing Embryonic Morphology using
Optical Coherence Tomography," Dev. Biol. 177: 54-64 (1996); S.A. Boppart, M.E. Brezinski, G.J. Tearney, B.E. Bouma, and J.G.
Fujimoto, "Imaging Developing Neural Morphology using Optical Coherence Tomography," J. Neurosci. Meth. 70: 65-72 (1996).
100 S.A. Boppart, G.J. Tearney, B.E. Bouma, M.E. Brezinski, J.F. Southern, and J.G. Fujimoto, "Noninvasive Assessment of the Developing Xenopus Cardiovascular System using Optical Coherence Tomography," Proc. Nat. Acad. Sci. 94: 4256-61 (1997).
101 S.A. Boppart, B.E. Bouma, C. Pitris, J.F. Southern, M.E. Brezinski, and J.G. Fujimoto, "In Vivo Subcellular Optical Coherence Tomography Imaging in Xenopus laevis: Implications for the Early Diagnosis of Neoplasms," submitted to J. Clinical Investigat.
125
Chapter 1. Optics and Quantum Electronics
Publications
Boppart, S.A., B.E. Bouma, C. Pitris, G.J. Tearney,
J.F. Southern, M.E. Brezinski, and J.G. Fujimoto.
"Three-dimensional Optical Coherence Tomography for Microsurgical Diagnostics." Radiol. Forthcoming.
Boppart, S.A., M.E. Brezinski, C. Pitris, and J.G.
Fujimoto. "Optical Coherence Tomography for
the Identification of Human Brain Tumors and
Margins." Submitted to Neurosurg.
Boppart, S.A., G.J. Tearney, B.E. Bouma, J.F. Southern, M.E. Brezinski, and J.G. Fujimoto. "Noninvasive Assessment of the Developing Xenopus
Cardiovascular System using Optical Coherence
Tomography." Proc. Nat. Acad. Sci. 94: 4256-61
(1997).
Boppart, S.A., B.E. Bouma, C. Pitris, G.J. Tearney,
and J.G Fujimoto. "Forward-scanning Instruments for Optical Coherence Tomographic Imaging." Opt. Lett. 22: 1618-20 (1997).
Boppart, S.A., B.E. Bouma, C. Pitris, J.F. Southern,
M.E. Brezinski, J.G. Fujimoto. "In Vivo, High
Speed Subcellular Imaging: Feasibility for the
Early Diagnosis of Neoplasms." Submitted to
Nature Med.
Bouma, B.E., L.E. Nelson, G.J. Tearney, D.J. Jones,
M.E. Brezinski, and J.G. Fujimoto. "Optical
Coherence Tomographic Imaging of Human Tissue at 1.55 mm and 1.8 mm using Er- and Tmdoped Fiber Sources." J. Biomed. Opt. Forthcoming.
Brezinski, M.E., G.J. Tearney, N.J. Weissman, S.A.
Boppart, B.E. Bouma, M.R. Hee, A.E. Weyman,
E.A. Swanson, J.F. Southern, and J.G. Fujimoto.
"Assessing Atherosclerotic Plaque Morphology:
Comparison of Optical Coherence Tomography
and High Frequency Intravascular Ultrasound."
Brit. Heart J. 77: 397-404 (1997).
Brezinski, M.E., G.J. Tearney, S.A. Boppart, E.A.
Swanson, J.F. Southern, and J.G Fujimoto.
"Optical Biopsy with Optical Coherence Tomography, Feasibility for Surgical Diagnostics." J.
Surgical Res. 71: 32-40 (1997)
Chinn, S.R., E.A. Swanson, and J.G. Fujimoto. "Optical Coherence Tomography using a Frequencytunable Optical Source." Opt. Lett. 22: 340-42
(1997).
DiCarlo, C.D., W.P. Roach, D.A. Gagliano, S.A. Boppart, D.X. Hammer, A.B. Cox, and J.G. Fujimoto,
"Comparison of Optical Coherence Tomography
(OCT) Imaging of Cataracts with Histopathlogy."
Submitted to Investigat. Ophthalmol. Visual Sci.
126 RLE Progress Report Number 140
Fujimoto, J.G., S.A. Boppart, G.J. Tearney, B.E.
Bouma, C. Pitris, and M.E. Brezinski. "High Resolution In Vivo Intraarterial Imaging with Optical
Coherence Tomography." Submitted to Circulat.
Herrmann, J.M., M.E. Brezinski, B.E. Bouma, S.A.
Boppart, C. Pitris, and J.G. Fujimoto. "Two and
Three Dimensional High Resolution Imaging of
the Human Oviduct with Optical Coherence
Tomography." Fertil. Steril. Forthcoming.
Herrmann, J.M., C. Pitris, B.E. Bouma, S.A. Boppart,
J.G. Fujimoto, and M.E. Brezinski. "High Resolution Imaging of Normal and Wsteoartheritic Cartilage with Optical Coherence Tomography."
Submitted to J. Rheumatol.
Narayan, D.G., C.A. Toth, S. Boppart, M.R. Hee, J.G.
Fujimoto, R. Birngruber, C.P. Cain, C.D. DiCarlo,
and W.P. Roach. "A Comparison of Retinal Morphology Viewed by Optical Coherence Tomography
and
Light
Microscopy."
Investigat.
Ophthalmol. Visual Sci. Forthcoming.
Tearney, G.J., M.E. Brezinski, J.F. Southern, B.E.
Bouma, S.A. Boppart, and J.G. Fujimoto. "Optical
Biopsy in Human Urologic Tissue using Optical
Coherence Tomography." J. Urol. 157: 1915-19
(1997).
Tearney, G.J., M.E. Brezinski, B.E. Bouma, S.A. Boppart, C. Pitris, J.F. Southern, and J.G. Fujimoto.
"In Vivo Endoscopic Optical Biopsy with Optical
Coherence Tomography." Sci. 276: 2037-39
(1997).
Tearney, G.J., M.E. Brezinski, J.F. Southern, B.E.
Bouma, S.A. Boppart, and J.G. Fujimoto. "Optical
Biopsy in Human Gastrointestinal Tissue using
Optical Coherence Tomography." Am. J. Gastroenterol. 92: 1800-04 (1997).
Tearney, G.J., B.E. Bouma, and J.G. Fujimoto. "Highspeed Phase- and Group-delay Scanning with a
Grating-based Phase Control Delay Line." Opt.
Lett. 22: 1811-13 (1997).
Tong, Y.P. , P.M. W. French, J.R. Taylor, and J.G.
Fujimoto. "All-solid-state Femtosecond Sources
in the Near Infrared." Opt. Commun. 136: 23538 (1997).
Toth, C.A., R. Birngruber, S.A. Boppart, M.R. Hee,
J.G. Fujimoto, C.D. DiCarlo, E.A. Swanson, C.P.
Cain, D. Narayan, G.D. Noojin, and W.P. Roach.
"Argon Retinal Laser Lesions Evaluated In Vivo
by Optical Coherence Tomography." Am. J. Ophthalmol. 123: 188-98 (1997).
Zuclich, J.A., ST. Schuschereba, H. Zwick, S.A.
Boppart, J.G. Fujimoto, F.E. Cheney, and B.E.
Stuck. "A Comparison of Laser-induced Retinal
Chapter 1. Optics and Quantum Electronics
Damage from Infrared Wavelengths to that from
Visible Wavelengths." Lasers Light Ophthalmol.
8:15-29 (1997).
Conference Presentations
Boppart, B.A., J.M. Herrmann, C. Pitris, B.E. Bouma,
G.J. Tearney, M.E. Brezinski, and J.G. Fujimoto.
"Interventional Optical Coherence Tomography
for Surgical Guidance." Submitted for presentation at the Conference on Lasers and Electro
Optics, CLEO98, San Francisco, California, May
1998.
Boppart, S.A., M.E. Brezinski, B.E. Bouma, C. Pitris,
J.F. Southern and J.G. Fujimoto. "Microsurgical
Guidance using Optical Coherence Tomography." Conference on Lasers and Electro Optics
CLEO'97, Baltimore, Maryland, May 18-23,
1997, paper CWD2.
Boppart, S.A., B.E. Bouma, C. Pitris, J.F. Southern,
M.E. Brezinski, and J.G. Fujimoto. "Optical
Coherence Tomographic Imaging of In Vivo Cellular Dynamics." Paper presented at the
Advances in Optical Imaging and Photon Migration Conference, Orlando, Florida, March 9-13,
1998.
Bouma, B.E., G.J. Tearney, S.A. Boppart, B. Golubovic, I.P. Bilinsky, and J.G. Fujimoto. "Mode
Locked Solid State Laser Sources for Optical
Tomography." Coherence Domain Optical Methods in Biomedical Science and Clinical Applications, BIOS 97 International Biomedical Optics
Symposium, SPIE, February 8-14, 1997, San
Jose, California.
Brezinski, M.E., G.J. Tearney, S.A. Boppart, B.E.
Bouma, N.J. Weissman, C. Pitris, J.G Fujimoto.
"Micron Scale Catheter Based In Vivo and In
Vitro Imaging with Optical Coherence Tomography." Oral presentation, Transcatheter Cardiovascular Therapeutics, Washington,
D.C.,
September 24-28, 1997.
Brezinski, M.E., J.M. Herrmann, C. Pitris, B.E.
Bouma, S.A. Boppart, J.F. Southern, J.G. Fujimoto. "Ultrahigh Resolution Imaging of Normal and
Osteoarthritic Cartilage Microstructure." Poster
presentation, American College of Rheumatology National Scientific Meeting, Washington,
D.C., November 8-12, 1997.
Brezinski, M.E., G.J. Tearney, S.A. Boppart, B.
Bouma, E.A. Swanson, J.F. Southern, and J.G.
Fujimoto. "In Vivo Imaging with Optical Coherence Tomography." American College of Cardiology 46th Annual Scientific Session, Anaheim,
California, March 16-19, 1997
Brezinski, M.E., B.E. Bouma, S.A. Boppart, C. Pitris,
J.F. Southern, and J.G. Fujimoto. "In Vivo High
Resolution Imaging of Gastrointestinal Tissue
using Optical Coherence Tomography." Paper
presented at the Annual Meeting of the American
Society for Gastrointestinal Endoscopy at Digestive Disease Week, Washington, D.C., May 1114, 1997.
Brezinski, M.E., C. Pitris, S.A. Boppart, G.J. Tearney,
J.F. Southern, and J.G. Fujimoto. "Micron Scale
Optical Biopsy with Optical Coherence Tomography." Submitted for presentation to ASCO Annual
Meeting, Denver, Colorado, May 17-20, 1997.
Chernikov, S.V., J.R. Taylor, V.P. Gapontsev, B.E.
Bouma, and J.G. Fujimoto. "A 75-nm, 30-mW
Superfluorescent Ytterbium Fiber Source Operating Around 10.6 mm." Conference on Lasers
and Electro Optics CLEO '97, Baltimore, Maryland, May 18-23, 1997, paper CTuG8.
Coker, J.G. M.R. Hee, C.A. Puliafito, J.C. Szwartz,
J.S. Duker, E. Reichel, J. S. Schuman, E.A.
Swanson, and J.G. Fujimoto. "Prognosis and
Anatomic Outcome of Macular Hole Surgery
Assessed by Optical Coherence Tomography."
Association for Research in Vision and Ophthalmology Annual Meeting, Fort Lauderdale, Florida, May 11-16, 1997, paper 3455-B56.
Fujimoto, J.G., G. Tearney, S. Boppart, C. Pitris, B.
Bouma, J. Southern, and M. Brezinski. "New
Techniques for High Resolution and High Speed
Optical Coherence Tomography." New York
Academy of Sciences and the Center for
Advanced Technology at CUNY on Advances in
Optical Biopsy and Optical Mammography, April
24-25, 1997, New York.
Fujimoto, J.G., B.E. Bouma, G.J. Tearney, S.A. Boppart, C. Pitris, J.F. Southern, and M.E. Brezinski.
"High Speed and High Resolution Optical Coherence Tomography using Femtosecond Lasers."
Conference on Lasers and Electro Optics
CLEO'97, May 18-23, 1997, Baltimore, Maryland, (invited) paper CTuS1.
Fujimoto, J.G., G.J. Tearney, S.A. Boppart, C. Pitris,
and B.E. Bouma. "High Speed High Resolution
Optical Coherence Tomography for Optical
Biopsy." LASERmed 97, 13th International Congress Laser Medicine, Munich, Germany, June
18-20, 1997.
Fujimoto, J.G., G.J. Tearney, S.A. Boppart, C. Pitris,
B.E. Bouma, J. Herrmann, J.F. Southern, and
M.E. Brezinski. "Optical Coherence Tomographic
in Medicine." Fifth Congress of the International
127
Chapter 1. Optics and Quantum Electronics
Society for Skin Imaging, Vienna, Austria, September 25-27, 1997, invited talk.
Fujimoto, J.G. "Optical Coherence Tomography for
Biomedical Imaging." The International Topical
Workshop on Contemporary Photonic Technologies, Tokyo, Japan, January 11-15, 1998, invited
talk.
Herrmann, J.M., C. Pitris, B.E. Bouma, S.A. Boppart,
J.G. Fujimoto, and M.E. Brezinski. "Two and
Three Dimensional Imaging of Normal and
Osteoarthritic Cartilage Microstructure with Optical Coherence Tomography." Paper presented at
the Advances in Optical Imaging and Photon
Migration Conference, Orlando, Florida, March
9-13, 1998.
Herrmann, J.M., S.A. Boppart, B.E. Bouma, G.J.
Tearney, C. Pitris, M.E. Brezinski, and J.G.
Fujimoto. "Real Time Imaging of Laser Intervention with Optical Coherence Tomography." Paper
presented at the Advances in Optical Imaging
and Photon Migration Conference, Orlando, Florida, March 9-13, 1998.
Huang, L.N., J.S. Schuman, T.P. Kloizman, L.
Pieroth, N. Wang, M.R. Hee, J.G. Coker, C.A.
Puliafito, J.G. Fujimoto, and E.A. Swanson. "The
Comparison of Nerve Fiber Layer Thickness in
Glaucomatous Monkey Eyes Measured by Optical Coherence Tomography." Association for
Research in Vision and Ophthalmology Annual
Meeting, Fort Lauderdale, Florida, May 11-16,
1997, paper 3913-B514.
Kim, V.Y, S. Roh, L. Pieroth, T. Pedut-Kloizman, M.
Hee, J.G. Coker, J.C. Szwartz, C.A. Puliafito, J.S.
Schuman, E.A. Swanson, J.G. Fujimoto, and J.S.
Duker. "Quantification of Nerve Fiber Layer and
Retinal Thickness using Optical Coherence
Tomography in Patients with Human ImmunodeAssociation
for
ficiency Virus Infection."
Research in Vision and Ophthalmology Annual
Meeting, Fort Lauderdale, Florida, May 11-16,
1997, paper 5151-B658.
Pieroth, L., J.S. Schuman, E. Hertzmark, M.R. Hee,
J.G.Coker, P.Y Chung, T Pedut-Kloizman, C.A.
Puliafito, C. Mattox, J.G. Fujimoto, and E.A.
Swanson. "Comparison of the Nerve Fiber Layer
by Optical Coherence Tomography Before and
After Intraocular Pressure Lowering Procedures." Association for Research in Vision and
Ophthalmology Annual Meeting, Fort Lauderdale, Florida, May 11-16, 1997, paper 3916B517.
Pitris, C., S.A. Boppart, B.E. Bouma, G. Tearney, J.G.
Fujimoto, and M.E. Brezinski. "High Resolution
128 RLE Progress Report Number 140
In-vivo Intravascular Imaging with Optical Coherence Tomography." Paper presented at the
Advances in Optical Imaging and Photon Migration Conference, Orlando, Florida, March 9-13,
1998.
Tearney, G.J., S.A. Boppart, B.E. Bouma, C. Pitris,
M.E. Brezinski, J.F. Southern, E. A. Swanson,
and J.G. Fujimoto. "High Speed Catheter/Endoscopic Optical Coherence Tomography for the
Optical Biopsy of in vivo Tissues." Conference on
Lasers and Electro Optics CLEO '97, Baltimore,
Maryland, May 18-23, 1997, paper CWD5.
Pitris, C., S.A. Boppart, B.E. Bouma, and J.G.
Fujimoto. "Cellular and Neoplastic Tissue Imaging with Optical Coherence Tomography." Submitted for presentation at the Conference on
Lasers and Electro Optics, CLEO98, San Francisco, California, May 1998.
Tearney, G.J., M.E. Brezinski, B.E. Bouma, J.F.
Southern, and J.G. Fujimoto. "High Resolution
Imaging of Urologic Tissues using Optical Coherence Tomography." American Urological Association 1997 Annual Meeting, New Orleans,
Louisiana, April 12-17, 1997, Abstract 479.
Tearney, G.J., B.E. Bouma, S.A. Boppart, M.E. Brezinski, J.F. Southern, E.A. Swanson, and J.G.
Fujimoto. "Endoscopic Optical Coherence Tomography." Tomography and Spectroscopy of Tissue, BIOS 97 International Biomedical Optics
Symposium, SPIE, San Jose, California, February 8-14, 1997,
Toth, C.A., D.G. Narayan, W.P. Roach, S.A. Boppart,
M.R. Hee, C.D. DiCarlo, E.A. Swanson, C.P.
Cain, G.D. Noojin, and J.G. Fujimoto. "Retinal
Injury Evaluated using Optical Coherence
Tomography." BIOS 97 International Biomedical
Optics Symposium, SPIE, San Jose, California,
February 8-14, 1997.
Toth, C.A., D.G. Narayan, W.P. Roach, S.A. Boppart,
M.R. Hee, J.G. Fujimoto, R. Birngruber, C.D.
DiCarlo, C.P. Cain, and G.D. Noojin. "Analyzing
Retinal Laser Effects: Old and New Techniques."
Conference on Lasers and Electro Optics CLEO
'97, Baltimore, Maryland, May 18-23, 1997,
(invited) paper CTuT1.
Chapter 1. Optics and Quantum Electronics
1.16 Analytical Confirmation of
Stochastic Soliton Formulation
Project Staff
In the second-quantized formulation, O(x,t) is the
operator analog of the field amplitude and evolves
according to
8
John M. Fini, Professor Peter L. Hagelstein and Professor Hermann A. Haus
Optical solitons have been demonstrated in regimes
useful for long-distance communications and the
generation of squeezed light. Quantum effects are of
fundamental importance in such systems, since the
dominant noise is often of quantum-mechanical origin. Previous work 2 has led to a good theoretical
understanding of soliton propagation and interactions, in agreement with experiment. 0 3
An alternative stochastic formulation of the same
quantum evolution was given by Drummond and
Carter early on,10 and has been used numerically to
evaluate squeezing. Until now, no direct comparison
has been made of results of the two formulations.
We have applied the stochastic theory to the computation of basic, interesting quantities-the spreading
of position and phase in a freely propagating soliton 10 5 (see Figure 20). The result is that the linearized
stochastic theory agrees exactly with predictions of
the operator theory. 10 6 This validates the stochastic
method and allows greater confidence in its further
application.
-0
1l
.
= i-+
t
c
cPK
2
2
)x
(1)
J
The basic equations of motion for the stochastic formalism govern the formal, complex variables 0 and
0 +. The evolution,
1
i 21
+co-0
2x
2,
2-O+a
-i2.
7
] 0+
+
0ic)' ""4(XM
+O
+ (-ic) /20+'+(x.t)
is, according to its derivation, exactly equivalent to
the operator equation.
Traditional soliton perturbation theory extracts solvable modes of the linearized equations of motion.
Adapting this basic principle to the stochastic differential equations (2) and (3), we can similarly project
out four special modes which are decoupled from all
other modes of the system. Having solved the SDEs
for these modes, we can then relate them to physical
variables of interest in accordance with the stochastic construction.
AX
A
Specifically, just as perturbation theory applied to
equation (1) gives the evolution of position and
momentum operators,
A
Sender
at
Receiver
Figure 20. This figure depicts accumulated
quantum uncertainty in the position of pulses in a
communications system. The accumulating
uncertainty in position, and thus in arrival time, of a
pulse places fundamental limitations on the
communication rate.
so do the stochastic equations give,
C\.
4
lch =
x~suc.
Cp. AtacJ
,toch+
noise
102 J.P. Gordon and H.A. Haus, Opt. Lett. 11: 665 (1986); Y. Lai and H.A. Haus, Phys. Rev. A 40: 844 (1989); Y. Lai and H.A. Haus, Phys.
Rev. A 40: 854 (1989); P.L. Hagelstein, Phys. Rev. A 54: 2426 (1996).
103 H.A. Haus and W.S. Wong, Rev Mod. Phys. 68: 423 (1996).
104 P.D. Drummond and S.J. Carter, J. Opt. Soc. Am. B 4:1565 (1987).
105 J.M. Fini, P.L. Hagelstein, and H.A. Haus, Phys. Rev A, forthcoming.
106 H.A. Haus and Y.Lai, J. Opt. Soc. Am. B7:386 (1990); Y. Lai, J. Opt. Soc. Am. B 10: 475 (1993); P.L. Hagelstein, Phys. Rev A 54:
2426 (1996).
129
Chapter 1. Optics and Quantum Electronics
Equation (5) and the corresponding evolutions for n,
p, and 0 constitute a linear system, easily solved in
terms of the complex noise drives. The variables
etc., represent the statistics of the quantum
cx.stoch,
operators x, p, etc., but only indirectly. To connect
quantum and stochastic statistics, we must carefully
follow the (somewhat counter-intuitive) rules prescribed by the Drummond and Carter construction. 10 7
The basic statistical correspondence behind the stochastic method looks deceptively simple:
,,)= (wp )..,f.0 )
(( ^-),II
(6)
are
We must be cautious because while p and
operators and are Hermitian conjugate to one
another, ( and 0,are complex numbers and are generally not conjugate. Care must be taken with the
ordering of operators. The functions 0 and 0' must
not be thought of as physical variables, except insofar as they generate physical moments through
equation (6). Practically, this means that the stochastic method involves traditional operator algebra as
well as stochastic manipulations.
In calculating variances of the soliton position and
phase, the correct correspondences have the form,
(Ax)
= (c
toch) + constant commutator
essential features of this calculation and the stochastic method can be captured in a simplified single-nonlinear-oscillator tutorial.
1.16.1 Publication
Fini, J.M., P.L. Hagelstein, and H.A. Haus. "Agreement of Stochastic Soliton Formalism with
Second-Quantized and Configuration-Space
Models." Phys. Rev A. Forthcoming.
1.17 Long-Time Evolution of Soliton
Position and Phase in the SecondQuantized Model
Project Staff
John M. Fini, Professor Peter L. Hagelstein, and Professor Hermann A. Haus
Quantum models of soliton propagation have been
used to understand noise and interaction of these
pulses in optical fiber. Recently, Hagelstein 08 has
demonstrated a simple and effective approach to
quantum optics problems; using a photon configuration-space model based on the familiar second-quantized theory,10 9 he has computed the long-time
evolution (beyond the linearization) of soliton position
and phase spread.
(7)
The stochastic moment, computed by solving the
system (2) and (3) is then combined with the commutator term resulting from operator algebra. Commutators give us the initial vacuum fluctuations of a
coherent state, while stochastic moments add subsequent deviations from coherent-state statistics in
quadrature. The result is in full agreement with previous results on the evolution of spreading in soliton
position, phase, momentum, and photon number.
The calculation we have done allows us to place
greater confidence in the stochastic method, whether
it is applied analytically or numerically. It serves as a
model calculation, bringing out several nonintuitive
aspects of the theory. Finally, we have found that all
While the use of this approach may have far-reaching implications for the more general use of photon
configuration space, the calculation itself does not
involve any radical new assumptions. To demonstrate this, we show that free-particle dynamics for
soliton position and phase are the result of the Hartree approximation and of large photon number. 110 By
rederiving these dynamics, we clarify the essential
features of the system and demonstrate that the calculation can be made entirely within the secondquantized framework. In particular, the issue of defining a quantum phase operator for solitons is
explored.
Soliton position and
dynamics. That is, they
with no applied force.
sponding to this motion
phase follow free particle
act as coordinates evolving
The intuitive picture correis essentially classical (Fig-
107 P.D. Drummond and S.J. Carter, J. Opt. Soc. Am. B 4:1565 (1987); C.W. Gardiner, Handbook of Stochastic Methods for Physics,
Chemistry, and the Natural Sciences (New York: Springer-Verlag, 1985).
108 P.L. Hagelstein, Phys. Rev. A 54: 2426 (1996).
109 Y. Lai and H.A. Haus, Phys. Rev. A 40: 844 (1989); Y. Lai and H.A. Haus, Phys. Rev A 40: 854 (1989).
110 J.M. Fini, PL. Hagelstein, and H.A. Haus, "Nonperturbative Calculation of Sloiton Variables." Submitted to Phys. Rev. A.
130 RLE Progress Report Number 140
Chapter 1. Optics and Quantum Electronics
ure 21). These dynamics are reflected in the
appearance of conserved, conjugate variables (number and momentum) in the linearized operator theory 11' and are clearly spelled out in the configuration
space calculation.
Px
Ap
AX
-X
S*
X
AX
Figure 21. The evolution in phase space of a
collection of free particles is illustrated. Spreading in
position results from either quantum-mechanical or
statistical mechanical evolution.
The case of position is simpler. Here we have the
exact relation:
X, H]= iPt,
M
operator A associated with the central-phase. With
infinitely many (x) s, it is not obvious a priori that
any special combination exists.
Using the Hartree approximation in the limit of large
photon number allows us to overcome these difficulties. On the one hand, a Hartree product state corresponding to a soliton has a well-defined "mode" that
all of the photons are in, allowing us to associate a
single A with the soliton. It then makes sense to
think of N and 0 as (approximately) conjugate variables,
[0,N]
i
On the other hand, there is a well-defined energy
associated with the number of a Hartree product
state, so that we can write the effective Hamiltonian
seen by 6,
(8)
Ho = hw(o)
[P,H] = 0
(9)
Hagelstein shows this by separating center-position
and relative position parts of the Hamiltonian. However, we have obtained these relations explicitly by
manipulating photon operators, consistent with the
definitions of Lai and Haus. 112 From here, Hagelstein's time-dependence of position spread is simply
obtained from the standard rules for evolving quantum expectations (in any picture):
(A')"t
(12)
While the issues relating to these two physical statements are complicated, we assert that they present a
correct and intuitive description for how soliton phase
should work.1 1 4 We will now show that they are all
that is needed to obtain free-particle dynamics fully
analogous to equations (8) and (9).
From the above assumptions, we obtain quite directly
[6,]= ih do
X
(11)
(13)
dN
M
(10)
+(zXkzP+APAX)t M±(AX ))
The case of phase spreading is not as simple. It is
not even clear that we can write down a valid phase
operator in terms of photon operators. The problem
of finding a valid phase operator for a single oscillator
has been studied for many years." 3 Satisfactory
results are possible in the limit of high photon number. An optical field is not a single oscillator, but
rather many operators. For solitons, we want to identify a single phase for this multimode field. This suggests a favored mode, or a special annihilation
[N.H] = 0
(14)
Equation (13) in essence identifies a "velocity" operator associated with 3, which is a function of N . The
second states that N, and thus also the "velocity," is
stationary. The two represent free-particle dynamics
in exactly the same way that equations (8) and (9)
do.
Finding the time-dependence of phase spread is
again only a matter of applying standard quantum
methods. The result, in the limit relevant to the soliton problem is
111 H.A. Haus and Y Lai, J. Opt. Soc. Am. B7:386 (1990); Y. Lai, J. Opt. Soc. Am. B 10: 475 (1993).
112 Y. Lai and H.A. Haus, Phys. Rev A 40: 844 (1989).
113 M.M. Nieto, Physica Scripta T48:5 (1993).
114 P.L. Hagelstein, Phys. Rev. A 54: 2426 (1996).
Chapter 1. Optics and Quantum Electronics
2
(A9 ) = (0),, + [o"(n,)] (An2),,t
(15)
The position operator of the ith atom is i , and the
. This has previously been
d
dN( n
shown to agree with the linearized theory of Haus
and Lai.
amplitude of the mth phonon mode is 4,,. The success of this point of view is so complete that by now
we think only in terms of one view or the other, all the
while recognizing that the two approaches are completely equivalent.
where co"(n)
,
=
By continuing the arguments of Hagelstein, we have
arrived at what is essential to the free-particle
dynamics of soliton position and phase. We have
demonstrated that these dynamics can be derived
entirely within the second-quantized formulation and
have identified the characteristics of solitons which
allow us to find a quantum phase operator. We now
have an obvious approach to identifying phase for
more general fields.
1.17.1 Publication
Fini, J.M., P.L. Hagelstein, and H.A. Haus. "Nonperturbative Calculation of Soliton Variables." Submitted to Phys. Rev A.
1.18 Local Consequences of Strong
Phonon Excitation in a Lattice
But what happens when the lattice is forced to exhibit
interesting behavior both on the local level and globally as well? Suppose we set up a problem in which
a delocalized phonon mode is very highly excited,
which necessitates a phonon point of view, and then
ask questions about the detailed local dynamics such
as the atom-atom correlation, which necessitates a
local position point of view. While the phonon point of
view allows for a convenient description of the global
dynamics, it is not convenient at all for treating that
atom-atom interaction on a small scale. Conversely,
while a position operator description is very good at
describing the local atom-atom correlations, the corresponding description of the global excitation
becomes highly inconvenient.
This discussion motivates the introduction of a novel
hybrid description, in which the global excitation is
described through the use of a single phonon amplitude, and in which the local dynamics are described
in terms of a residual position operator. We propose
the hybrid construction 1
Project Staff
Professor Peter L. Hagelstein
1.18.1 Hybrid Lattice Description
i = Ri + uiq.
We are familiar with two descriptions of the motion of
atoms in a lattice. We can describe the position of the
atom in terms of the center of mass location of the
nucleus. This would be appropriate for describing
energetic two-body interactions, such as the dislodging of an atom in a lattice by an MeV neutron. Alternatively, we can describe the local atomic dynamics
in terms of the vibrational modes of the lattice. Such
a description would be useful in analyzing the
response of an atom and its surroundings in the case
of low energy transfer, as might occur in the case of
Bragg scattering of a neutron from a crystal. These
two different descriptions are ultimately completely
equivalent, as described through the quantum relation
The residual local position operator is I , which is
made up of the contributions of all the other phonon
modes and which behaves very much like a local
position operator. In the absence of large excitation
in the phonon mode that is singled out, results
obtained with the residual operator would be indistinguishable from results obtained using the true position operator when the lattice is large. A schematic of
this construction is given in Figure 22.
(16)
RR + =R')
ou(
m)
.
115 P.L. Hagelstein. "Atom-atom Correlation in the Presence of Strong Terahertz Phonon Excitation," Phil. Mag. B, forthcoming.
132
RLE Progress Report Number 140
(17)
Chapter 1. Optics and Quantum Electronics
uq
second order. This means that for all low-order interactions with atoms in the lattice that our classical
intuition is basically sound. For example, in a gentle
collision of a neutron with an atom in the lattice, the
classical picture will give the correct answer on average for energy transfer. This is not to say that on
closer inspection there are not quantum effects that
show up, for surely there are. Instead, we have an
assurance that on average each atom responds as if
it had an energy on average on the order of EL/N. It is
never the case that the total lattice energy somehow
shows up at one atomic site in such low-order interactions.
Within the quantum description outlined above, the
excitation of the lattice with the total lattice energy EL
appears explicitly in the description of the atomic
position. For example, in writing
Figure 22. Schematic of contributions of the highly
excited phonon mode and residual position operator
to the atomic position in a lattice.
1.18.2 Quantum Mechanical Versus Classical
View
If a phonon mode is highly excited, we usually picture
the resulting dynamics in our mind's eye in terms of a
classical picture. We see atoms as masses and the
interaction with other atoms in terms of springs (very
small springs, but springs nonetheless). The motion
on the atomic scale is that of an atom vibrating synchronously with other atoms, oscillating at a characteristic frequency that may be on the order of a few
terahertz in the example that interests us here. In our
classical picture, the more highly excited the lattice
is, the larger the local amplitude of vibrations. If the
total vibrational energy of the lattice is EL, then we
imagine that each individual atom has an energy that
is on the order of
E
i = Ri
,+uq
(19)
we observe that the phonon coordinate q is part of a
many-particle quantum system with a total energy
that can be 50 MeV in our example. Each atom, in
fact, does know about the total lattice energy of the
excited mode; whether any observable effects ever
arise is another story altogether. Certainly it can be
shown that no significant energy exchange from the
highly excited mode can occur through first order
interactions that are low order in q . But whether
there exist first order interactions that are very high
order in q or second order interactions that behave
differently must be determined by direct computation
or by experiment. This is presently an open question,
one that has only just been posed in our work.
1.18.3 Impact of Strong Phonon Excitation
on Local Correlations
(18)
Any interaction that might exhibit interesting new
effects in which a larger fraction of the total lattice
where N is the number of atoms in the lattice. For
example, if the average local atomic energy is Ea = 5
meV, and if there are N = 101, atoms present (we
imagine here a small lattice), then the total lattice
energy EL is 50 MeV-a very large total energy for
the many-atom lattice.
energy will be involved must be highly nonlinear in q.
Consequently, we seek any very strong interactions
that are highly nonlinear in the atomic positions as
candidates that may show new physics. One obvious
candidate is the Coulomb interaction at short range
between the atomic nuclei of neighboring atoms.
When two atoms approach to within 50 fermis of one
another, the associated Coulomb interaction is very
strong (the associated potential energy is about 300
keV for protons), and the interaction is exceedingly
E
Nl
The quantum description of the lattice is not very different in most respects from the classical description.
Ehrenfest's theorem ensures that the dynamics of
the average position operator matches that of the
classical positions for all local potential terms up to
nonlinear in q . Of course, it is a rare occurrence that
133
Chapter 1. Optics and Quantum Electronics
two atoms will ever be so close, but when they are
we might expect that the resulting dynamics may be
sensitive to the total lattice energy in some way.
To investigate this, we work with a hybrid version of
the relative atom-atom displacement operator
ARii
where ARlii is the residual atom-atom displacement
in the absence of the highly excited mode. Any new
effects will show up in modifications of the lattice
wavefunction, which we might approximate with the
hybrid description in terms of
(21)
(R,, R1, q).
The total lattice wavefunction T on the left knows in
detail about the dynamics of all the atoms in the lat(R, R, q) focuses our
tice; the hybrid version
attention on the highly excited mode and on the local
residual dynamics of the two atoms. The relevant
Schrodinger equation is then of the form
(24)
-R
1
If the phonon mode degree of freedom were separable from the local degrees of freedom
(20)
= ARi + Au q,
-_
h
T= 0(q)y(Ri, R),
(25)
then the computation of the reaction rate is straightforward, and this model leads to significant rates of
lattice-induced recoil. The inclusion of correlation
between the local degrees of freedom and the
phonon mode amplitude as discussed above complicates matters and results in a significant reduction of
the predicted recoil emission rates from the separated result. We are pursuing this to try to obtain reliable estimates of the rate for this effect.
1.19 Steady-State Hydrodynamic
Ablation
Project Staff
Professor Peter L. Hagelstein, Susan Sujono
hV2
h2V2
2M
2M
+- MIo
q
h
2Mq aq
+ V(R i,R
= FE'
)
Approximate solutions of this equation can be developed according to
P = O(q)y(Ri, R1 q),
(23)
in the general spirit of the Born-Oppenheimer
approximation. This approach is presently under
investigation.
1.18.4 Possibility of Lattice-Induced Recoil
We are interested in the possibility that some of the
energy in the highly excited phonon mode may be
transferred to the local degrees of freedom. We are
imagining that the lattice is highly excited so that
energy is readily available, and we are interested in
the high order nonlinearity of the Coulomb potential
at close range which we hope will mediate the
energy transfer. The computation of reaction rates for
this process involving protons for example follows
from Fermi's Golden Rule
In many x-ray laser experiments, an intense laser is
incident on a solid surface in line focus, and hot
plasma is ablated. We are interested in the temperature and density profile in order to understand
whether the plasma so created is suitable for the
development of gain in the EUV and soft x-ray
regimes. While the plasma evolution is transient, it is
of interest to understand the steady state limit of the
ablation, most interestingly because it represents an
upper limit of the capability of the plasma to conduct
heat from the critical surface to the low-density
region where gain is expected.
We have explored both one-dimensional and two
dimensional versions of the problem."16 The cylindrical expansion model in one dimension provides a
very good approximation for the two-dimensional
problem. For this case, we seek solutions of the coupled hydrodynamic equations
ld
r dr
r drVp =
d
Vd\
(26)
d0
d
r drP
(27)
116 S. Sujono, Analytical Study of Steady State Plasma Ablation from Soft X-ray Laser Target, M.Eng. thesis, Department of Electrical
Engineering and Computer Science, MIT, 1997.
134 RLE Progress Report Number 140
Chapter 1. Optics and Quantum Electronics
vdT
dr
=
2 T Id
I(rv)
3 rdr
+ 211d(
2 I
d T\ .
-T
3Nrdr
dr
(28)
Temperature and velocity profiles have been computed numerically. We have also investigated the use
of analytic models, and we have found that accurate
normalized velocity and temperature profiles can be
developed from few parameter fits according to
u(4) = 1 + (x(l
- e "
(4) = e -
,
)
(29)
(30)
.
Here u is the plasma velocity in terms of the sound
speed at the critical surface, and r is the temperature
normalized to the critical surface value. The variable
is the logarithmic spatial coordinate In(r/ro) where ro
is the radius associated with the critical surface.
These results are illustrated in Figure 23.
2
Dashed: exact
Dotted: analytic
r, u
1
0
5
10
15
20
r/ro
Figure 23. Numerical solution and analytic
solutions for u and -i for steady state ablation in
cylindrical geometry.
1.19.1 Thesis
Sujono, S. Analytical Study of Steady State Plasma
Ablation from Soft X-ray Laser Target. M.Eng
thesis, Department of Electrical Engineering and
Computer Science, MIT, 1997.
136
RLE Progress Report Number 140